doc: Improve some grammar and sentences

90fbf7c has taken care of that for HEAD.  This includes the portion of
the fixes that applies to the documentation, where needed depending on
the branch.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20201227202604.GC26311@telsasoft.com
Backpatch-through: 9.5

Author: michaelpq

doc/src/sgml/ref/pg_dump.sgml

@@ -626,7 +626,7 @@ PostgreSQL documentation
       <listitem>
        <para>
         Specify the compression level to use.  Zero means no compression.
-        For the custom archive format, this specifies compression of
+        For the custom and directory archive formats, this specifies compression of
         individual table-data segments, and the default is to compress
         at a moderate level.
         For plain text output, setting a nonzero compression level causes

doc/src/sgml/sources.sgml

@@ -103,7 +103,7 @@ less -x4
     message text.  In addition there are optional elements, the most
     common of which is an error identifier code that follows the SQL spec's
     SQLSTATE conventions.
-    <function>ereport</function> itself is just a shell macro, that exists
+    <function>ereport</function> itself is just a shell macro that exists
     mainly for the syntactic convenience of making message generation
     look like a single function call in the C source code.  The only parameter
     accepted directly by <function>ereport</function> is the severity level.