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

doc/src/sgml/acronyms.sgml

@@ -408,6 +408,26 @@
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><acronym>MCF</acronym></term>
+    <listitem>
+     <para>
+      Most Common Frequency, that is the frequency associated with some
+      Most Common Value
+     </para>
+    </listitem>
+   </varlistentry>
+
+   <varlistentry>
+    <term><acronym>MCV</acronym></term>
+    <listitem>
+     <para>
+      Most Common Value, one of the values appearing most often within a
+      particular table column
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><acronym>MITM</acronym></term>
     <listitem>

doc/src/sgml/planstats.sgml

@@ -203,7 +203,7 @@ EXPLAIN SELECT * FROM tenk1 WHERE stringu1 = 'xxx';
    <acronym>MCV</acronym>s:
 
 <programlisting>
-selectivity = (1 - sum(mvf))/(num_distinct - num_mcv)
+selectivity = (1 - sum(mcv_freqs))/(num_distinct - num_mcv)
             = (1 - (0.00333333 + 0.003 + 0.003 + 0.003 + 0.003 + 0.003 +
                     0.003 + 0.003 + 0.003 + 0.003))/(676 - 10)
             = 0.0014559