Small docs bugfix: make it clear what can be used in UPDATE FROM andDELETE USING

Hello,

Currently the documentation says that one can put "a list of table 
expressions"
after FROM in UPDATE or after USING in DELETE.
However, "table expression" is defined as a complex of
FROM, WHERE, GROUP BY and HAVING clauses [1].
The thing one can list in the FROM clause in a comma-separated manner
is called a table reference [2].
SELECT reference does not use this term but explains what they could be [3].

Please could someone have a look at the patch attached?
It's not just pedantry but rather based on a real-life example of 
someone reading and being not sure
whether e.g. joins can be used in there.

Best, Alex

[1] https://www.postgresql.org/docs/devel/queries-table-expressions.html
[2] 
https://www.postgresql.org/docs/devel/queries-table-expressions.html#QUERIES-FROM
[3] https://www.postgresql.org/docs/devel/sql-select.html#SQL-FROM