qemu-options.hx: Update descriptions of memory options for NUMA node

qemu-options.hx: Update descriptions of memory options for NUMA node

This commit adds the following description:
1. `memdev` option is recommended over `mem` option (see [1,2])
2. users must specify memory for all NUMA nodes (see [2])

This commit also separates descriptions for `mem` and `memdev` into two
paragraphs. The old doc describes legacy `mem` option first, and it was
a bit confusing.

Related documentation:
[1] https://wiki.qemu.org/ChangeLog/5.1#Incompatible_changes
[2] https://www.qemu.org/docs/master/about/removed-features.html

Signed-off-by: Yohei Kojima <y-koj@outlook.jp>
Message-Id: <TYZPR06MB5418D6B0175A49E8E76988439D8E9@TYZPR06MB5418.apcprd06.prod.outlook.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
[AJB: fix documentation in commit message]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230424092249.58552-15-alex.bennee@linaro.org>