[PATCH] clarify palloc comment on quote_literal_cstr

Hello hackers,

I found the numbers in `quote_literal_cstr` palloc quite magical. So I've
added a comment clarifying what they mean. The change is small:

/* We make a worst-case result area; wasting a little space is OK */
- result = palloc(len * 2 + 3 + 1);
+ result = palloc(
+ (len * 2) /* worst-case doubling for every character if
each one is a quote */
+ + 3 /* two outer quotes + possibly 'E' if needed */
+ + 1 /* null terminator */
+ );

Best regards,
Steve