doc: Reword unexplained abbreviation

The previous wording used MVF to indicate the Most Common Values'
Frequencies, but the abbreviation was never explained or defined.
Reword to mcv_freqs to make the use clearer.

Also add MCF and MCV as acronyms as they were using <acronym>
markup but were missing from the acronyms page.

Reported-by: Eric Mutta <eric.mutta@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/166112292492.654.5377188452604176150@wrigleys.postgresql.org

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