Fix some grammar and typos in comments and docs

The documentation fixes are backpatched down to where they apply.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20201031020801.GD3080@telsasoft.com
Backpatch-through: 9.6

doc/src/sgml/auto-explain.sgml

@@ -200,7 +200,7 @@ LOAD 'auto_explain';
     <listitem>
      <para>
       <varname>auto_explain.log_settings</varname> controls whether information
-      about modified configuration options is printed when execution plan is logged.
+      about modified configuration options is printed when an execution plan is logged.
       Only options affecting query planning with value different from the built-in
       default value are included in the output.  This parameter is off by default.
       Only superusers can change this setting.

doc/src/sgml/ddl.sgml

@@ -3919,7 +3919,7 @@ CREATE INDEX ON measurement (logdate);
 
     <para>
       Normally the set of partitions established when initially defining the
-      table are not intended to remain static.  It is common to want to
+      table is not intended to remain static.  It is common to want to
       remove old partitions of data and periodically add new partitions for
       new data. One of the most important advantages of partitioning is
       precisely that it allows this otherwise painful task to be executed

doc/src/sgml/ref/pg_rewind.sgml

@@ -74,7 +74,7 @@ PostgreSQL documentation
    <application>pg_rewind</application> with the <literal>-c</literal> option to
    automatically retrieve them from the WAL archive. The use of
    <application>pg_rewind</application> is not limited to failover, e.g.,  a standby
-   server can be promoted, run some write transactions, and then rewinded
+   server can be promoted, run some write transactions, and then rewound
    to become a standby again.
   </para>