Mention warning and error message docs in PR template
Mention warning and error message docs in PR template

This closes #7580

c.f. https://github.com/coq/coq/pull/7559#issuecomment-390749207 and
https://github.com/coq/coq/pull/7559#issuecomment-390872924.  This
should be reverted if and when we move to autogenerated docs for
warnings and errors, as suggested in #7373.

silene/coq,gares/coq,gares/coq,maximedenes/coq,maximedenes/coq,Matafou/coq,maximedenes/coq,SkySkimmer/coq,gares/coq,ppedrot/coq,ejgallego/coq,coq/coq,Zimmi48/coq,ejgallego/coq,SkySkimmer/coq,silene/coq,Matafou/coq,ppedrot/coq,SkySkimmer/coq,ejgallego/coq,ppedrot/coq,coq/coq,gares/coq,Matafou/coq,silene/coq,maximedenes/coq,ppedrot/coq,maximedenes/coq,JasonGross/coq,SkySkimmer/coq,ppedrot/coq,gares/coq,Zimmi48/coq,coq/coq,JasonGross/coq,Zimmi48/coq,Zimmi48/coq,JasonGross/coq,ejgallego/coq,JasonGross/coq,coq/coq