doc: Add acronym and glossary term for Access Method

AM was used throughout the documentation to denote Access Method, but
the acronym was not described.  This adds an acronym entry as well as
a glossary term which the acronym links to.  Each page which describe
AMs have the first occurrence with <acronym> markup.

Reported-by: alaa.attya91@gmail.com
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/169974408805.398198.6927340566912872957@wrigleys.postgresql.org

doc/src/sgml/acronyms.sgml

@@ -9,6 +9,15 @@
 
   <variablelist>
 
+   <varlistentry>
+    <term><acronym>AM</acronym></term>
+    <listitem>
+     <para>
+      <link linkend="glossary-am">Access Method</link>
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><acronym>ANSI</acronym></term>
     <listitem>

doc/src/sgml/btree.sgml

@@ -27,7 +27,7 @@
   and understanding of sorting semantics.  Therefore, they've acquired
   some features that go beyond what would be needed just to support btree
   indexes, and parts of the system that are quite distant from the
-  btree AM make use of them.
+  btree <acronym>AM</acronym> make use of them.
  </para>
 
 </sect1>

doc/src/sgml/glossary.sgml

@@ -39,6 +39,21 @@
    </glossdef>
   </glossentry>
 
+  <glossentry id="glossary-am">
+   <glossterm>Access Method</glossterm>
+   <glossdef>
+    <para>
+     Interfaces which <productname>PostgreSQL</productname> use in order to
+     access data in tables and indexes.  This abstraction allows for adding
+     support for new types of data storage.
+    </para>
+    <para>
+     For more information, see <xref linkend="tableam" /> and
+     <xref linkend="indexam" />.
+    </para>
+   </glossdef>
+  </glossentry>
+
   <glossentry>
    <glossterm>Analytic function</glossterm>
    <glosssee otherterm="glossary-window-function" />

doc/src/sgml/indexam.sgml

@@ -215,7 +215,7 @@ typedef struct IndexAmRoutine
    the first; however they are permitted to require some restriction to
    appear for the first index column, and this is signaled by setting
    <structfield>amoptionalkey</structfield> false.
-   One reason that an index AM might set
+   One reason that an index <acronym>AM</acronym> might set
    <structfield>amoptionalkey</structfield> false is if it doesn't index
    null values.  Since most indexable operators are
    strict and hence cannot return true for null inputs,

doc/src/sgml/tableam.sgml

@@ -56,7 +56,7 @@
 
  <para>
   To implement an access method, an implementor will typically need to
-  implement an AM-specific type of tuple table slot (see
+  implement an <acronym>AM</acronym>-specific type of tuple table slot (see
   <ulink url="https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/include/executor/tuptable.h;hb=HEAD">
    <filename>src/include/executor/tuptable.h</filename></ulink>), which allows
    code outside the access method to hold references to tuples of the AM, and