docs: don't use :hg: at the beginning of lines in notes (issue3397)

It seem like docutils 0.8 interpret ':hg:`command`' roles at the beginning of
indented lines in '.. note::' directives as a field that is an invalid argument
to the directive. It fails with 'Error in "note" directive: invalid option
block.' Docutils 0.7 accepted this arguably incorrect markup.

Reflowing the text makes the problem go away. A leading '\ ' could perhaps also
be used to mask the problem.

mercurial/help/patterns.txt

@@ -7,8 +7,8 b' patterns.'
 Alternate pattern notations must be specified explicitly.
 
 .. note::
-  Patterns specified in ``.hgignore`` are not rooted. Please see
-  :hg:`help hgignore` for details.
+  Patterns specified in ``.hgignore`` are not rooted. 
+  Please see :hg:`help hgignore` for details.
 
 To use a plain path name without any pattern matching, start it with
 ``path:``. These path names must completely match starting at the