Update section on debug specifier to include !r conversion

[davepeck]

As mentioned by Jan Kaliszewski in Discourse:

to be consistent with f-strings, it should be treated as t'expr={expr!r}'

That's a good catch!

[zuo]

To be even more precise:

• t'{expr=}' should be treated as t'expr={expr!r}', but
• t'{expr=:SOMETHING}' should be treated as t'expr={expr!s:SOMETHING}' (in particular: t'{expr=:}' should effectively be treated as t'expr={expr!s}', and
• whitespace characters before expr as well as before and after = should be kept as-is [assuming it is technically possible].