add tab-complete for memory, serialize option and other minor issues.

hi.

I found some minor issues related to the EXPLAIN command.

cannot auto-complete with a white space.
src8=# explain (analyze,b

can auto-complete:
src8=# explain (analyze, b

to make tab-complete work, comma, must be followed with a white space,
not sure why.
--------------
explain (serialize binary) select 1;
ERROR:  EXPLAIN option SERIALIZE requires ANALYZE

do you think it's better to rephrase it as:
ERROR:  EXPLAIN option SERIALIZE requires ANALYZE option

since we have separate ANALYZE SQL commands.
--------------

     
      Specify the output format, which can be TEXT, XML, JSON, or YAML.
      Non-text output contains the same information as the text output
      format, but is easier for programs to parse.  This parameter defaults to
      TEXT.
     

should we add  attribute for {TEXT, XML, JSON, YAML} in the
above paragraph?

--------------
i created a patch for tab-complete for memory, SERIALIZE option.