doc: Fix inconsistencies in GIN, BRIN and SP-GiST for optional opclass methods

The GIN and SP-GiST parts were out-of-sync since the changes of 14903f23,
and the BRIN part was wrong since its introduction in 15cb2bd2.

Author: Guillaume Lelarge
Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/CAECtzeXKvEPEr967h0PRYRi39uTmdEms=oUtc_PWGjZRNN1prw@mail.gmail.com
Backpatch-through: 13

doc/src/sgml/brin.sgml

@@ -585,7 +585,7 @@ typedef struct BrinOpcInfo
 
       <para>
        Since both key extraction of indexed values and representation of the
-       key in <acronym>GIN</acronym> are flexible, they may depend on
+       key in <acronym>BRIN</acronym> are flexible, they may depend on
        user-specified parameters.
       </para>
      </listitem>

doc/src/sgml/gin.sgml

@@ -379,7 +379,7 @@
  </para>
 
  <para>
-  Optionally, an operator class for <acronym>GIN</acronym> can supply the
+  An operator class for <acronym>GIN</acronym> can optionally supply the
   following methods:
 
   <variablelist>
@@ -406,7 +406,7 @@
      <term><function>void options(local_relopts *relopts)</function></term>
      <listitem>
       <para>
-       Defines set of user-visible parameters that control operator class
+       Defines a set of user-visible parameters that control operator class
        behavior.
       </para>
 

doc/src/sgml/spgist.sgml

@@ -881,7 +881,7 @@ typedef struct spgLeafConsistentOut
      <term><function>options</function></term>
      <listitem>
       <para>
-       Defines set of user-visible parameters that control operator class
+       Defines a set of user-visible parameters that control operator class
        behavior.
       </para>