Call out vacuum considerations in create index docs

Backpatch to pg12, which is as far as it goes without conflicts.

Author: James Coleman <jtc331@gmail.com>
Reviewed-by: "David G. Johnston" <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/CAAaqYe9oEfbz7AxXq7OX+FFVi5w5p1e_Of8ON8ZnKO9QqBfmjg@mail.gmail.com

doc/src/sgml/ref/create_index.sgml

@@ -823,6 +823,12 @@ Indexes:
    to remove an index.
   </para>
 
+  <para>
+   Like any long-running transaction, <command>CREATE INDEX</command> on a
+   table can affect which tuples can be removed by concurrent
+   <command>VACUUM</command> on any other table.
+  </para>
+
   <para>
    Prior releases of <productname>PostgreSQL</productname> also had an
    R-tree index method.  This method has been removed because

doc/src/sgml/ref/reindex.sgml

@@ -404,6 +404,12 @@ Indexes:
     CONCURRENTLY</command> cannot.
    </para>
 
+   <para>
+    Like any long-running transaction, <command>REINDEX</command> on a table
+    can affect which tuples can be removed by concurrent
+    <command>VACUUM</command> on any other table.
+   </para>
+
    <para>
     <command>REINDEX SYSTEM</command> does not support
     <command>CONCURRENTLY</command> since system catalogs cannot be reindexed