Tidy up a few cosmetic issues related to pg_basebackup.

Commit 0ad8032 failed to update
the pg_basebackup documentation to mention that "client-" or
"server-" can now be prepended to the compression method name. Fix
it there, and also in the --help output that you get from running
the binary.

Also in the documentation, there's an old issue that the arguments to
--checkpoint shouldn't be marked as parameters, because "fast" and
"spread" are literal strings. Fix that too.

Dagfinn Ilmari Mannsåker, partly as per a report from
Shinoda Noriyoshi.

Discussion: http://postgr.es/m/PH7PR84MB1885C1CF433057807551172BEE5F9@PH7PR84MB1885.NAMPRD84.PROD.OUTLOOK.COM

Author: robertmhaas

doc/src/sgml/ref/pg_basebackup.sgml

@@ -398,9 +398,9 @@ PostgreSQL documentation
 
      <varlistentry>
       <term><option>-Z <replaceable class="parameter">level</replaceable></option></term>
-      <term><option>-Z <replaceable class="parameter">method</replaceable></option>[:<replaceable>level</replaceable>]</term>
+      <term><option>-Z [{client|server}-]<replaceable class="parameter">method</replaceable></option>[:<replaceable>level</replaceable>]</term>
       <term><option>--compress=<replaceable class="parameter">level</replaceable></option></term>
-      <term><option>--compress=[[{<replaceable class="parameter">client|server</replaceable>-}]<replaceable class="parameter">method</replaceable></option>[:<replaceable>level</replaceable>]</term>
+      <term><option>--compress=[{client|server}-]<replaceable class="parameter">method</replaceable></option>[:<replaceable>level</replaceable>]</term>
       <listitem>
        <para>
         Requests compression of the backup. If <literal>client</literal> or
@@ -441,8 +441,8 @@ PostgreSQL documentation
 
     <variablelist>
      <varlistentry>
-      <term><option>-c <replaceable class="parameter">fast|spread</replaceable></option></term>
-      <term><option>--checkpoint=<replaceable class="parameter">fast|spread</replaceable></option></term>
+      <term><option>-c {fast|spread}</option></term>
+      <term><option>--checkpoint={fast|spread}</option></term>
       <listitem>
        <para>
         Sets checkpoint mode to fast (immediate) or spread (the default)

src/bin/pg_basebackup/pg_basebackup.c

@@ -391,7 +391,7 @@ usage(void)
 	printf(_("  -X, --wal-method=none|fetch|stream\n"
 			 "                         include required WAL files with specified method\n"));
 	printf(_("  -z, --gzip             compress tar output\n"));
-	printf(_("  -Z, --compress={gzip,none}[:LEVEL] or [LEVEL]\n"
+	printf(_("  -Z, --compress={[{client,server}-]gzip,none}[:LEVEL] or [LEVEL]\n"
 			 "                         compress tar output with given compression method or level\n"));
 	printf(_("\nGeneral options:\n"));
 	printf(_("  -c, --checkpoint=fast|spread\n"