doc: Add better description for rewrite functions in event triggers

There are two functions that can be used in event triggers to get more
details about a rewrite happening on a relation.  Both had a limited
documentation:
- pg_event_trigger_table_rewrite_reason() and
pg_event_trigger_table_rewrite_oid() were not mentioned in the main
event trigger section in the paragraph dedicated to the event
table_rewrite.
- pg_event_trigger_table_rewrite_reason() returns an integer which is a
bitmap of the reasons why a rewrite happens.  There was no explanation
about the meaning of these values, forcing the reader to look at the
code to find out that these are defined in event_trigger.h.

While on it, let's add a comment in event_trigger.h where the
AT_REWRITE_* are defined, telling to update the documentation when
these values are changed.

Backpatch down to 13 as a consequence of 1ad23335f3, where this area
of the documentation has been heavily reworked.

Author: Greg Sabino Mullane
Discussion: https://postgr.es/m/CAKAnmmL+Z6j-C8dAx1tVrnBmZJu+BSoc68WSg3sR+CVNjBCqbw@mail.gmail.com
Backpatch-through: 13

src/include/commands/event_trigger.h

@@ -29,6 +29,12 @@ typedef struct EventTriggerData
 	CommandTag	tag;
 } EventTriggerData;
 
+/*
+ * Reasons for relation rewrites.
+ *
+ * pg_event_trigger_table_rewrite_reason() uses these values, so make sure to
+ * update the documentation when changing this list.
+ */
 #define AT_REWRITE_ALTER_PERSISTENCE	0x01
 #define AT_REWRITE_DEFAULT_VAL			0x02
 #define AT_REWRITE_COLUMN_REWRITE		0x04