doc/hgrc.5.txt: add anchors for sections
Enables links into hgrc.5.html of the form: hgrc.5.html#section
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'