doc: Reorganize section for shared memory and LWLocks.

doc: Reorganize section for shared memory and LWLocks.

Presently, this section meanders through a few different features,
and the text itself is terse.  This commit attempts to improve
matters by splitting the section into smaller sections and by
expanding the text for clarity.  This is preparatory work for a
follow-up commit that will introduce a way for libraries to use
shared memory without needing to request it at startup time.

Reviewed-by: Aleksander Alekseev, Bharath Rupireddy, Abhijit Menon-Sen
Discussion: https://postgr.es/m/20240112041430.GA3557928%40nathanxps13
Discussion: https://postgr.es/m/20231205034647.GA2705267%40nathanxps13