doc: Update SQL keywords for SQL:2016 technical corrigenda

Several keywords were missing or misclassified in the original
SQL:2016 standard.  This has been corrected in later technical
corrigenda.  This change updates the PostgreSQL documentation
accordingly.

This also fixes a mistake in 606948b: The keywords JSON_SCALAR and
JSON_SERIALIZE added there are not from SQL:2016 but from future
SQL:202x, so they don't belong in that list yet.

(606948b also added JSON to the reserved list, which is what the
corrigendum also does, but failed to remove it from the nonreserved
list.)

Author: petere

doc/src/sgml/keywords/sql2016-02-nonreserved.txt

@@ -100,7 +100,6 @@ INSTANTIABLE
 INSTEAD
 INVOKER
 ISOLATION
-JSON
 K
 KEEP
 KEY
@@ -115,6 +114,7 @@ M
 MAP
 MATCHED
 MAXVALUE
+MEASURES
 MESSAGE_LENGTH
 MESSAGE_OCTET_LENGTH
 MESSAGE_TEXT
@@ -131,10 +131,12 @@ NFD
 NFKC
 NFKD
 NORMALIZED
+NULL_ORDERING
 NULLABLE
 NULLS
 NUMBER
 OBJECT
+OCCURRENCE
 OCTETS
 OPTION
 OPTIONS
@@ -158,11 +160,14 @@ PASS
 PASSING
 PAST
 PATH
+PERMUTE
+PIPE
 PLACING
 PLAN
 PLI
 PRECEDING
 PRESERVE
+PREV
 PRIOR
 PRIVATE
 PRIVILEGES
@@ -196,13 +201,15 @@ SCOPE_SCHEMA
 SECTION
 SECURITY
 SELF
+SEMANTICS
 SEQUENCE
 SERIALIZABLE
 SERVER_NAME
 SESSION
 SETS
 SIMPLE
 SIZE
+SORT_DIRECTION
 SOURCE
 SPACE
 SPECIFIC_NAME
@@ -232,6 +239,7 @@ UNBOUNDED
 UNCOMMITTED
 UNCONDITIONAL
 UNDER
+UNMATCHED
 UNNAMED
 USAGE
 USER_DEFINED_TYPE_CATALOG

doc/src/sgml/keywords/sql2016-02-reserved.txt

@@ -1,4 +1,5 @@
 ABS
+ABSENT
 ACOS
 ALL
 ALLOCATE
@@ -163,8 +164,6 @@ JSON_EXISTS
 JSON_OBJECT
 JSON_OBJECTAGG
 JSON_QUERY
-JSON_SCALAR
-JSON_SERIALIZE
 JSON_TABLE
 JSON_TABLE_PRIMITIVE
 JSON_VALUE