issue_template: explain manpages link
issue_template: explain manpages link

This tries to clarify why we're sending people to the "community" page
for problems with the manpage content.

mosoft521/gitscm-next,Mokolea/git-scm.com,Mokolea/git-scm.com,Mokolea/git-scm.com,mosoft521/gitscm-next,Mokolea/git-scm.com,mosoft521/gitscm-next,git/git-scm.com,git/git-scm.com,git/git-scm.com,git/git-scm.com,mosoft521/gitscm-next