doc: clarify what to do with Go 1.4 when installing from source
doc: clarify what to do with Go 1.4 when installing from source

You have to actually run make.bash (or make.bat).

Update #18771.

Change-Id: Ie6672a4e4abde0150c1ae57cabb1222de2c78716
Reviewed-on: https://go-review.googlesource.com/35632
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,golang/go,akutz/go,Cofyc/go,ImJasonH/go,vsekhar/elastic-go,cloudflare/go,cloudflare/go,ImJasonH/go,vsekhar/elastic-go,nawawi/go,vsekhar/elastic-go,nawawi/go,codestation/go,ImJasonH/go,cloudflare/go,golang/go,nawawi/go,josharian/go,golang/go,akutz/go,akutz/go,golang/go,vsekhar/elastic-go,Cofyc/go,josharian/go,codestation/go,Cofyc/go,CAFxX/go,cloudflare/go,linux-on-ibm-z/go,josharian/go,CAFxX/go,cloudflare/go,vsekhar/elastic-go,akutz/go,CAFxX/go,derekmarcotte/go,CAFxX/go,akutz/go,polyverse-security/go,polyverse-security/go,nawawi/go,codestation/go,ImJasonH/go,Cofyc/go,akutz/go,Cofyc/go,polyverse-security/go,polyverse-security/go,derekmarcotte/go,cloudflare/go,linux-on-ibm-z/go,Cofyc/go,linux-on-ibm-z/go,cloudflare/go,polyverse-security/go,CAFxX/go,derekmarcotte/go,vsekhar/elastic-go,linux-on-ibm-z/go,josharian/go,ImJasonH/go,codestation/go,codestation/go,nawawi/go,derekmarcotte/go,polyverse-security/go,codestation/go,derekmarcotte/go,derekmarcotte/go,vsekhar/elastic-go,ImJasonH/go,nawawi/go,josharian/go,polyverse-security/go,golang/go,linux-on-ibm-z/go,CAFxX/go,linux-on-ibm-z/go,cloudflare/go,akutz/go,CAFxX/go,derekmarcotte/go,derekmarcotte/go,linux-on-ibm-z/go,golang/go,ImJasonH/go,akutz/go,josharian/go,CAFxX/go,Cofyc/go,christopher-henderson/Go,josharian/go,codestation/go,golang/go,josharian/go,golang/go,linux-on-ibm-z/go,Cofyc/go,polyverse-security/go,vsekhar/elastic-go,nawawi/go,codestation/go,ImJasonH/go