The current doc generation doesn't care much about indentation levels,
but we would like to switch to an rST format, and rST does care about
indentation.

Make the doc comments more strongly consistent about indentation
for multiline constructs like:

@arg: description line 1
      description line 2

Returns: line one
         line 2

so that there is always exactly one space after the colon, and
subsequent lines align with the first.

This commit is a purely whitespace change, and it does not alter the
generated .texi files (because the texi generation code strips away
all the extra whitespace).  This does mean that we end up with some
over-length lines.

Note that when the documentation for an argument fits on a single
line like this:

@arg: one line only

then stray extra spaces after the ':' don't affect the rST output, so
I have not attempted to methodically fix them, though the preference
is a single space here too.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200213175647.17628-10-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>