Clarify error message in Mahara syntax checker about table names with {}

[Aaron Wells]

I keep getting false positives from this syntax check:

        if (/\b(from|join)\s[^({]/i) {
            bad_line("this sql table is not surrounded by curly braces {}, ".
                     "or is not an anonymous inner select", $_);
        }

That's because it kicks in if you have comments including the very common preposition "from", or less frequently the word "join". I'm going to change the text in the Makefile to clarify what's triggering the message.

[Aaron Wells (u-aaronw)]

https://reviews.mahara.org/#/c/2388/