suggestion: add link to Emacs manual node `Bugs' in report-emacs-bug instructions

[Drew Adams]

Seems like we are forever repeating the excellent instructions found in the
Emacs manual node `Bugs' (and subnodes) to users in help-gnu-emacs. How about
including a link to that node in the `*mail*' buffer's instructions for
`report-emacs-bug'?

That is, instead of just saying "Please describe exactly what actions triggered
the bug and the precise symptoms of the bug", we give them a link to the manual
so they can see just what is recommended for such a description. Many users
could use some guidelines about what kinds of information are most helpful and
just what is meant by "precise symptoms".