user-manual: edit "ignoring files" for conciseness

user-manual: edit "ignoring files" for conciseness

The immediate motivation for writing this section was to explain the
various places ignore patterns could be used.  However, I still think
.gitignore is the case most people will want to learn about first.  It
also makes it a bit more concrete to introduce ignore patterns in the
context of .gitignore first.  And the existance of gitignore(5) relieves
the pressure to explain it all here.

So, stick to the .gitignore example, with only a brief mention of the
others, explain the syntax only by example, and leave the rest to
gitignore(5).

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Johan Herland <johan@herland.net>