doc: Update SQL keywords for SQL:2016 technical corrigenda

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 606948b058d: 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.

(606948b058d also added JSON to the reserved list, which is what the
corrigendum also does, but failed to remove it from the nonreserved
list.)