doc: itemize text blocks to increase readability in HTML format

Before this patch, text blocks changed in this patch are shown as just
continuous text blocks like below in HTML format.

    Global configuration like the username setting is typically put into:

    %USERPROFILE%\mercurial.ini
    $HOME/.hgrc

This patch itemizes these text blocks to increase readability in HTML
format.

    Global configuration like the username setting is typically put into:

    - %USERPROFILE%\mercurial.ini (on Windows)
    - $HOME/.hgrc (on Unix, Plan9)

Like as other platform sensitive container-ed text blocks, this patch
also adds explicit "on PLATFORM" information to each items for
readability in HTML format, even though output of "hg help config" on
command line seems a little redundant. For example, on Unix:

    Global configuration like the username setting is typically put into:

    - "$HOME/.hgrc" (on Unix, Plan9)

mercurial/help/config.txt

@@ -38,11 +38,11 b' Global configuration like the username s'
 
 .. container:: windows
 
-  ``%USERPROFILE%\mercurial.ini``
+  - ``%USERPROFILE%\mercurial.ini`` (on Windows)
 
 .. container:: unix.plan9
 
-  ``$HOME/.hgrc``
+  - ``$HOME/.hgrc`` (on Unix, Plan9)
 
 The names of these files depend on the system on which Mercurial is
 installed. ``*.rc`` files from a single directory are read in