doc: Update ALTER COLLATION wording

The original text on refreshing collation versions was written
specifically for ICU, and then information for other providers was
incrementally tacked on at the end.  Reword this to be a bit more
general and less reflective of how it was added.

Author: petere

doc/src/sgml/ref/alter_collation.sgml

@@ -104,9 +104,9 @@ ALTER COLLATION <replaceable>name</replaceable> SET SCHEMA <replaceable>new_sche
   <title>Notes</title>
 
   <para>
-   When using collations provided by the ICU library, the ICU-specific version
-   of the collator is recorded in the system catalog when the collation object
-   is created.  When the collation is used, the current version is
+   When a collation object is created, the provider-specific version of the
+   collation is recorded in the system catalog.  When the collation is used,
+   the current version is
    checked against the recorded version, and a warning is issued when there is
    a mismatch, for example:
 <screen>
@@ -117,21 +117,24 @@ HINT:  Rebuild all objects affected by this collation and run ALTER COLLATION pg
    A change in collation definitions can lead to corrupt indexes and other
    problems because the database system relies on stored objects having a
    certain sort order.  Generally, this should be avoided, but it can happen
-   in legitimate circumstances, such as when
+   in legitimate circumstances, such as when upgrading the operating system
+   to a new major version or when
    using <command>pg_upgrade</command> to upgrade to server binaries linked
    with a newer version of ICU.  When this happens, all objects depending on
    the collation should be rebuilt, for example,
    using <command>REINDEX</command>.  When that is done, the collation version
    can be refreshed using the command <literal>ALTER COLLATION ... REFRESH
    VERSION</literal>.  This will update the system catalog to record the
-   current collator version and will make the warning go away.  Note that this
+   current collation version and will make the warning go away.  Note that this
    does not actually check whether all affected objects have been rebuilt
    correctly.
   </para>
   <para>
    When using collations provided by <literal>libc</literal>, version
    information is recorded on systems using the GNU C library (most Linux
-   systems), FreeBSD and Windows.
+   systems), FreeBSD and Windows.  When using collations provided by ICU, the
+   version information is provided by the ICU library and is available on all
+   platforms.
   </para>
   <note>
    <para>