DOC: in-memory documentation

Author: Liudmila Mantrova

doc/src/sgml/contrib.sgml

@@ -124,6 +124,7 @@ CREATE EXTENSION <replaceable>module_name</> FROM unpackaged;
  &fuzzystrmatch;
  &hstore;
  &hunspell-dict;
+ &in-memory;
  &intagg;
  &intarray;
  &isn;

doc/src/sgml/filelist.sgml

@@ -128,6 +128,7 @@
 <!ENTITY fulleq          SYSTEM "fulleq.sgml">
 <!ENTITY hstore          SYSTEM "hstore.sgml">
 <!ENTITY hunspell-dict   SYSTEM "hunspell-dict.sgml">
+<!ENTITY in-memory       SYSTEM "in-memory.sgml">
 <!ENTITY intagg          SYSTEM "intagg.sgml">
 <!ENTITY intarray        SYSTEM "intarray.sgml">
 <!ENTITY isn             SYSTEM "isn.sgml">