docs: Clarify --with-segsize-blocks documentation

Without the added "relation" it's not immediately clear that the option
relates to the relation segment size and not e.g. the WAL segment size.

The option was added in d3b111e32.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/837536.1695348498@sss.pgh.pa.us
Backpatch: 16-

doc/src/sgml/installation.sgml

@@ -1681,7 +1681,7 @@ build-postgresql:
        <term><option>--with-segsize-blocks=SEGSIZE_BLOCKS</option></term>
        <listitem>
         <para>
-         Specify the segment size in blocks. If both
+         Specify the relation segment size in blocks. If both
          <option>--with-segsize</option> and this option are specified, this
          option wins.
 
@@ -3178,7 +3178,7 @@ ninja install
        <term><option>-Dsegsize_blocks=SEGSIZE_BLOCKS</option></term>
        <listitem>
         <para>
-         Specify the segment size in blocks. If both
+         Specify the relation segment size in blocks. If both
          <option>-Dsegsize</option> and this option are specified, this option
          wins.