Fix style nits in HeaderMap.java.
Fix style nits in HeaderMap.java.

Summary:
Use `checkArgument()` instead of `checkState()`, where appropriate.
Use doc comments instead of inline comments, where appropriate.
Reworded some documentation.
Fixed some indents.
Added a trailing comma for an ultimate enum value.

Test Plan: Sandcastle

Addepar/buck,clonetwin26/buck,OkBuilds/buck,shs96c/buck,zhuxiaohao/buck,davido/buck,brettwooldridge/buck,pwz3n0/buck,k21/buck,zpao/buck,clonetwin26/buck,brettwooldridge/buck,rowillia/buck,luiseduardohdbackup/buck,Addepar/buck,clonetwin26/buck,LegNeato/buck,pwz3n0/buck,mogers/buck,grumpyjames/buck,janicduplessis/buck,clonetwin26/buck,rowillia/buck,romanoid/buck,mikekap/buck,dpursehouse/buck,darkforestzero/buck,romanoid/buck,OkBuilds/buck,Addepar/buck,OkBuilds/buck,dsyang/buck,rmaz/buck,illicitonion/buck,romanoid/buck,marcinkwiatkowski/buck,romanoid/buck,shs96c/buck,rhencke/buck,vine/buck,mikekap/buck,grumpyjames/buck,justinmuller/buck,janicduplessis/buck,romanoid/buck,OkBuilds/buck,Dominator008/buck,raviagarwal7/buck,ilya-klyuchnikov/buck,bocon13/buck,bocon13/buck,janicduplessis/buck,dsyang/buck,zhan-xiong/buck,brettwooldridge/buck,robbertvanginkel/buck,dushmis/buck,shs96c/buck,liuyang-li/buck,Dominator008/buck,JoelMarcey/buck,siddhartharay007/buck,artiya4u/buck,vschs007/buck,stuhood/buck,grumpyjames/buck,zhuxiaohao/buck,k21/buck,liuyang-li/buck,mikekap/buck,mikekap/buck,pwz3n0/buck,mikekap/buck,1yvT0s/buck,mikekap/buck,robbertvanginkel/buck,rhencke/buck,Heart2009/buck,raviagarwal7/buck,shybovycha/buck,LegNeato/buck,ilya-klyuchnikov/buck,dsyang/buck,rhencke/buck,MarkRunWu/buck,bocon13/buck,liuyang-li/buck,dushmis/buck,sdwilsh/buck,Learn-Android-app/buck,romanoid/buck,rmaz/buck,marcinkwiatkowski/buck,mikekap/buck,nguyentruongtho/buck,Dominator008/buck,OkBuilds/buck,janicduplessis/buck,rhencke/buck,luiseduardohdbackup/buck,clonetwin26/buck,pwz3n0/buck,ilya-klyuchnikov/buck,rmaz/buck,LegNeato/buck,marcinkwiatkowski/buck,dsyang/buck,rmaz/buck,raviagarwal7/buck,nguyentruongtho/buck,ilya-klyuchnikov/buck,Dominator008/buck,JoelMarcey/buck,brettwooldridge/buck,justinmuller/buck,tgummerer/buck,shs96c/buck,rowillia/buck,clonetwin26/buck,shs96c/buck,JoelMarcey/buck,luiseduardohdbackup/buck,darkforestzero/buck,hgl888/buck,zhan-xiong/buck,justinmuller/buck,liuyang-li/buck,darkforestzero/buck,vine/buck,siddhartharay007/buck,daedric/buck,lukw00/buck,robbertvanginkel/buck,nguyentruongtho/buck,MarkRunWu/buck,dushmis/buck,siddhartharay007/buck,grumpyjames/buck,hgl888/buck,LegNeato/buck,illicitonion/buck,kageiit/buck,janicduplessis/buck,nguyentruongtho/buck,MarkRunWu/buck,shybovycha/buck,sdwilsh/buck,tgummerer/buck,ilya-klyuchnikov/buck,LegNeato/buck,dushmis/buck,bocon13/buck,kageiit/buck,justinmuller/buck,1yvT0s/buck,shybovycha/buck,clonetwin26/buck,JoelMarcey/buck,darkforestzero/buck,Distrotech/buck,robbertvanginkel/buck,davido/buck,grumpyjames/buck,neonichu/buck,artiya4u/buck,Addepar/buck,romanoid/buck,daedric/buck,siddhartharay007/buck,vine/buck,daedric/buck,dsyang/buck,mikekap/buck,liuyang-li/buck,daedric/buck,Learn-Android-app/buck,marcinkwiatkowski/buck,Dominator008/buck,Heart2009/buck,SeleniumHQ/buck,clonetwin26/buck,neonichu/buck,rmaz/buck,dsyang/buck,k21/buck,LegNeato/buck,JoelMarcey/buck,lukw00/buck,1yvT0s/buck,ilya-klyuchnikov/buck,romanoid/buck,1yvT0s/buck,illicitonion/buck,neonichu/buck,sdwilsh/buck,kageiit/buck,rowillia/buck,zhan-xiong/buck,LegNeato/buck,daedric/buck,rmaz/buck,Learn-Android-app/buck,rhencke/buck,dushmis/buck,robbertvanginkel/buck,neonichu/buck,justinmuller/buck,ilya-klyuchnikov/buck,zpao/buck,dushmis/buck,bocon13/buck,daedric/buck,raviagarwal7/buck,brettwooldridge/buck,Heart2009/buck,mogers/buck,mikekap/buck,stuhood/buck,rowillia/buck,mogers/buck,marcinkwiatkowski/buck,robbertvanginkel/buck,1yvT0s/buck,hgl888/buck,clonetwin26/buck,robbertvanginkel/buck,Addepar/buck,stuhood/buck,daedric/buck,JoelMarcey/buck,OkBuilds/buck,stuhood/buck,SeleniumHQ/buck,davido/buck,neonichu/buck,janicduplessis/buck,vine/buck,brettwooldridge/buck,shybovycha/buck,ilya-klyuchnikov/buck,zhuxiaohao/buck,zhuxiaohao/buck,shybovycha/buck,grumpyjames/buck,rowillia/buck,davido/buck,pwz3n0/buck,darkforestzero/buck,SeleniumHQ/buck,marcinkwiatkowski/buck,Heart2009/buck,stuhood/buck,tgummerer/buck,illicitonion/buck,tgummerer/buck,marcinkwiatkowski/buck,k21/buck,stuhood/buck,hgl888/buck,zhuxiaohao/buck,justinmuller/buck,shs96c/buck,pwz3n0/buck,raviagarwal7/buck,tgummerer/buck,k21/buck,romanoid/buck,darkforestzero/buck,raviagarwal7/buck,Dominator008/buck,vine/buck,zpao/buck,OkBuilds/buck,rowillia/buck,Learn-Android-app/buck,shybovycha/buck,shs96c/buck,robbertvanginkel/buck,OkBuilds/buck,clonetwin26/buck,mogers/buck,daedric/buck,brettwooldridge/buck,brettwooldridge/buck,Learn-Android-app/buck,SeleniumHQ/buck,romanoid/buck,facebook/buck,raviagarwal7/buck,Learn-Android-app/buck,nguyentruongtho/buck,SeleniumHQ/buck,illicitonion/buck,artiya4u/buck,luiseduardohdbackup/buck,zhan-xiong/buck,Addepar/buck,rhencke/buck,justinmuller/buck,lukw00/buck,sdwilsh/buck,grumpyjames/buck,marcinkwiatkowski/buck,daedric/buck,artiya4u/buck,Dominator008/buck,Distrotech/buck,stuhood/buck,romanoid/buck,raviagarwal7/buck,luiseduardohdbackup/buck,romanoid/buck,k21/buck,bocon13/buck,dpursehouse/buck,Addepar/buck,neonichu/buck,raviagarwal7/buck,facebook/buck,just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