docs: Add acronym and glossary entries for I/O and AIO

These are fairly basic, but better than nothing.  While there are several
opportunities to link to these entries, this patch does not add any. They will
however be referenced by future patches.

Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20250326183102.92.nmisch@google.com

doc/src/sgml/acronyms.sgml

@@ -9,6 +9,15 @@
 
   <variablelist>
 
+   <varlistentry>
+    <term><acronym>AIO</acronym></term>
+    <listitem>
+     <para>
+      <link linkend="glossary-aio">Asynchronous <acronym>I/O</acronym></link>
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><acronym>ACL</acronym></term>
     <listitem>
@@ -354,6 +363,15 @@
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><acronym>I/O</acronym></term>
+    <listitem>
+     <para>
+      <link linkend="glossary-io">Input/Output</link>
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><acronym>ISO</acronym></term>
     <listitem>

doc/src/sgml/glossary.sgml

@@ -81,6 +81,31 @@
    </glossdef>
   </glossentry>
 
+  <glossentry id="glossary-aio">
+   <glossterm>Asynchronous <acronym>I/O</acronym></glossterm>
+   <acronym>AIO</acronym>
+   <indexterm>
+    <primary>Asynchronous <acronym>I/O</acronym></primary>
+   </indexterm>
+   <glossdef>
+    <para>
+     Asynchronous <acronym>I/O</acronym> (<acronym>AIO</acronym>) describes
+     performing <acronym>I/O</acronym> in a non-blocking way (asynchronously),
+     in contrast to synchronous <acronym>I/O</acronym>, which blocks for the
+     entire duration of the <acronym>I/O</acronym>.
+    </para>
+    <para>
+     With <acronym>AIO</acronym>, starting an <acronym>I/O</acronym> operation
+     is separated from waiting for the result of the operation, allowing
+     multiple <acronym>I/O</acronym> operations to be initiated concurrently,
+     as well as performing <acronym>CPU</acronym> heavy operations
+     concurrently with <acronym>I/O</acronym>. The price for that increased
+     concurrency is increased complexity.
+    </para>
+    <glossseealso otherterm="glossary-io" />
+   </glossdef>
+  </glossentry>
+
   <glossentry id="glossary-atomic">
    <glossterm>Atomic</glossterm>
    <glossdef>
@@ -938,6 +963,20 @@
    </glossdef>
   </glossentry>
 
+  <glossentry id="glossary-io">
+   <glossterm>Input/Output</glossterm>
+   <acronym>I/O</acronym>
+   <glossdef>
+    <para>
+     Input/Output (<acronym>I/O</acronym>) describes the communication between
+     a program and peripheral devices. In the context of database systems,
+     <acronym>I/O</acronym> commonly, but not exclusively, refers to
+     interaction with storage devices or the network.
+    </para>
+    <glossseealso otherterm="glossary-aio" />
+   </glossdef>
+  </glossentry>
+
   <glossentry id="glossary-insert">
    <glossterm>Insert</glossterm>
    <glossdef>