Add some clarification for some files under config/*
Add some clarification for some files under config/*

I didn't add a lot of them because I am not very familiar about how they
work and I am not sure if we really need to explain files like
config/database.yml because if you know Rails you must know how
that works.

At any rate, I think this is a beginning and we could keep documenting
more files in the future.

Closes #18373

axilleas/gitlabhq,axilleas/gitlabhq,dplarson/gitlabhq,axilleas/gitlabhq,t-zuehlsdorff/gitlabhq,t-zuehlsdorff/gitlabhq,stoplightio/gitlabhq,darkrasid/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,darkrasid/gitlabhq,stoplightio/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,t-zuehlsdorff/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,openwide-java/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,darkrasid/gitlabhq,openwide-java/gitlabhq,iiet/iiet-git,htve/GitlabForChinese,openwide-java/gitlabhq,htve/GitlabForChinese,mmkassem/gitlabhq,dreampet/gitlab,htve/GitlabForChinese,jirutka/gitlabhq,iiet/iiet-git,openwide-java/gitlabhq,dplarson/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,htve/GitlabForChinese,dplarson/gitlabhq,iiet/iiet-git,dreampet/gitlab,mmkassem/gitlabhq,LUMC/gitlabhq,LUMC/gitlabhq,iiet/iiet-git,LUMC/gitlabhq,darkrasid/gitlabhq,dplarson/gitlabhq,LUMC/gitlabhq