doc: Add unit to pg_shmem_allocations attributes

The unit of size and allocated_size was not documented.  Speciyfing the
unit is in line with how many other (but not all) system view attributes
are documented so fixing by adding the unit which is "bytes".

Author: Euler Taveira <euler@eulerto.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reported-by: coleman.rik@gmail.com
Discussion: https://postgr.es/m/166033703458.653.1583077816076994614@wrigleys.postgresql.org

Author: danielgustafsson

doc/src/sgml/system-views.sgml

@@ -3485,7 +3485,7 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
        <structfield>size</structfield> <type>int8</type>
       </para>
       <para>
-       Size of the allocation
+       Size of the allocation in bytes
       </para></entry>
      </row>
 
@@ -3494,7 +3494,7 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
        <structfield>allocated_size</structfield> <type>int8</type>
       </para>
       <para>
-       Size of the allocation including padding. For anonymous
+       Size of the allocation in bytes including padding. For anonymous
        allocations, no information about padding is available, so the
        <literal>size</literal> and <literal>allocated_size</literal> columns
        will always be equal. Padding is not meaningful for free memory, so