doc: Clarify what files pg_basebackup omits from data directory

doc/src/sgml/protocol.sgml

@@ -1995,6 +1995,11 @@ The commands accepted in walsender mode are:
          <filename>postmaster.opts</>
         </para>
        </listitem>
+       <listitem>
+        <para>
+         various temporary files created during the operation of the PostgreSQL server
+        </para>
+       </listitem>
        <listitem>
         <para>
          <filename>pg_xlog</>, including subdirectories. If the backup is run
@@ -2003,6 +2008,18 @@ The commands accepted in walsender mode are:
          backup to work, not the rest of the contents.
         </para>
        </listitem>
+       <listitem>
+        <para>
+         <filename>pg_replslot</> is copied as an empty directory.
+        </para>
+       </listitem>
+       <listitem>
+        <para>
+         Files other than regular files and directories, such as symbolic
+         links and special device files, are skipped.  (Symbolic links
+         in <filename>pg_tblspc</filename> are maintained.)
+        </para>
+       </listitem>
       </itemizedlist>
       Owner, group and file mode are set if the underlying file system on
       the server supports it.

doc/src/sgml/ref/pg_basebackup.sgml

@@ -571,8 +571,10 @@ PostgreSQL documentation
   <para>
    The backup will include all files in the data directory and tablespaces,
    including the configuration files and any additional files placed in the
-   directory by third parties. Only regular files and directories are allowed
-   in the data directory, no symbolic links or special device files.
+   directory by third parties. But only regular files and directories are
+   copied.  Symbolic links (other than those used for tablespaces) and special
+   device files are skipped.  (See <xref linkend="protocol-replication"> for
+   the precise details.)
   </para>
 
   <para>