Clarify comment for worst-case allocation in quote_literal_cstr()

Clarify comment for worst-case allocation in quote_literal_cstr()

palloc() is invoked with a specific formula for its allocation size in
quote_literal_cstr().  This wastes some memory, but the size is large
enough to cover even the worst-case scenarios.

No explanations were given about the reasons behind these numbers.  This
commit adds more documentation about all that.

Author: Steve Chavez <steve@supabase.io>
Discussion: https://postgr.es/m/CAGRrpzZ9bToRWS+fAnjxDJrxwZN1QcJ-y1Pn2yg=Hst6rydLtw@mail.gmail.com