docs/devel/qapi-code-gen.txt: Update to new rST backend conventions

docs/devel/qapi-code-gen.txt: Update to new rST backend conventions

Update the documentation of QAPI document comment syntax to match
the new rST backend requirements. The principal changes are:
 * whitespace is now significant, and multiline definitions
   must have their second and subsequent lines indented to
   match the first line
 * general rST format markup is permitted, not just the small
   set of markup the old texinfo generator handled. For most
   things (notably bulleted and itemized lists) the old format
   was the same as rST is.
 * Specific things that might trip people up:
   - instead of *bold* and _italic_ rST has **bold** and *italic*
   - lists need a preceding and following blank line
   - a lone literal '*' will need to be backslash-escaped to
     avoid a rST syntax error
 * the old leading '|' for example (literal text) blocks is
   replaced by the standard rST '::' literal block.
 * we support arbitrary levels of sub- and sub-sub-heading, not
   just a main and sub-heading like the old texinfo generator
 * lists can now be nested

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-18-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Commit message improved slightly]
Signed-off-by: Markus Armbruster <armbru@redhat.com>