Spell "startup process" with lower case in the documentation

Most uses were already lower case, so this just makes all user-visible
spellings consistent.

Bharath Rupireddy

The proposed patch also had analagous changes for the code comments,
but I decided that wasn't worth the churn.

Discussion:
https://www.postgresql.org/message-id/flat/CALj2ACW7%2Bv_0QBPoWB%3DqKr67JKC019Htm%3DX8sKewS17bOquefg%40mail.gmail.com

doc/src/sgml/high-availability.sgml

@@ -2127,7 +2127,7 @@ HINT:  You can then restart the server after making the necessary configuration
    <para>
     <function>pg_cancel_backend()</function>
     and <function>pg_terminate_backend()</function> will work on user backends,
-    but not the Startup process, which performs
+    but not the startup process, which performs
     recovery. <structname>pg_stat_activity</structname> does not show
     recovering transactions as active. As a result,
     <structname>pg_prepared_xacts</structname> is always empty during
@@ -2139,9 +2139,9 @@ HINT:  You can then restart the server after making the necessary configuration
    <para>
     <structname>pg_locks</structname> will show locks held by backends,
     as normal. <structname>pg_locks</structname> also shows
-    a virtual transaction managed by the Startup process that owns all
+    a virtual transaction managed by the startup process that owns all
     <literal>AccessExclusiveLocks</literal> held by transactions being replayed by recovery.
-    Note that the Startup process does not acquire locks to
+    Note that the startup process does not acquire locks to
     make database changes, and thus locks other than <literal>AccessExclusiveLocks</literal>
     do not show in <structname>pg_locks</structname> for the Startup
     process; they are just presumed to exist.