contrib/bloom documentation improvement

This commit documents rounding of "length" parameter and absence of support
for unique indexes and NULLs searching.  Backpatch to 9.6 where contrib/bloom
was introduced.

Discussion: https://postgr.es/m/CAF4Au4wPQQ7EHVSnzcLjsbY3oLSzVk6UemZLD1Sbmwysy3R61g%40mail.gmail.com
Author: Oleg Bartunov with minor editorialization by me
Backpatch-through: 9.6

doc/src/sgml/bloom.sgml

@@ -51,8 +51,8 @@
     <term><literal>length</literal></term>
     <listitem>
      <para>
-      Length of each signature (index entry) in bits. The default
-      is <literal>80</literal> bits and maximum is <literal>4096</literal>.
+      Length of each signature (index entry) in bits, it is rounded up to the nearest
+      multiple of 16. The default is <literal>80</literal> bits and maximum is <literal>4096</literal>.
      </para>
     </listitem>
    </varlistentry>
@@ -242,6 +242,20 @@ DEFAULT FOR TYPE text USING bloom AS
       operations in the future.
      </para>
     </listitem>
+
+    <listitem>
+     <para>
+       <literal>bloom</literal> access method doesn't support
+       <literal>UNIQUE</literal> indexes.
+     </para>
+    </listitem>
+
+    <listitem>
+     <para>
+       <literal>bloom</literal> access method doesn't support searching for
+       <literal>NULL</literal> values.
+     </para>
+    </listitem>
    </itemizedlist>
   </para>
  </sect2>