doc: Mention BRIN indexes support parallel builds

Two places in the documentation suggest B-tree is the only index access
method allowing parallel builds. Commit b4375717 added parallel builds
for BRIN too, but failed to update the docs. So fix that, and backpatch
to 17, where parallel BRIN builds were introduced.

Author: Egor Rogov
Backpatch-through: 17
Discussion: https://postgr.es/m/114e2d5d-125e-07d8-94aa-5ad175fb7443@postgrespro.ru

doc/src/sgml/config.sgml

@@ -2810,7 +2810,7 @@ include_dir 'conf.d'
          Sets the maximum number of parallel workers that can be
          started by a single utility command.  Currently, the parallel
          utility commands that support the use of parallel workers are
-         <command>CREATE INDEX</command> only when building a B-tree index,
+         <command>CREATE INDEX</command> when building a B-tree or BRIN index,
          and <command>VACUUM</command> without <literal>FULL</literal>
          option.  Parallel workers are taken from the pool of processes
          established by <xref linkend="guc-max-worker-processes"/>, limited

doc/src/sgml/ref/create_index.sgml

@@ -808,7 +808,7 @@ Indexes:
    leveraging multiple CPUs in order to process the table rows faster.
    This feature is known as <firstterm>parallel index
    build</firstterm>.  For index methods that support building indexes
-   in parallel (currently, only B-tree),
+   in parallel (currently, B-tree and BRIN),
    <varname>maintenance_work_mem</varname> specifies the maximum
    amount of memory that can be used by each index build operation as
    a whole, regardless of how many worker processes were started.