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.
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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.
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<para>
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
<literal>TEXT</literal>.
</para>

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

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