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

src/backend/utils/adt/quote.c

@@ -108,7 +108,12 @@ quote_literal_cstr(const char *rawstr)
 
 	len = strlen(rawstr);
 	/* We make a worst-case result area; wasting a little space is OK */
-	result = palloc(len * 2 + 3 + 1);
+	result = palloc(
+					(len * 2)	/* doubling for every character if each one is
+								 * a quote */
+					+ 3			/* two outer quotes + possibly 'E' if needed */
+					+ 1			/* null terminator */
+		);
 
 	newlen = quote_literal_internal(result, rawstr, len);
 	result[newlen] = '\0';