Make it clearer what means 'won't work' for .if string == ${VAR}.

Replace the use of '=' in conditionals in the examples
by the more correct '=='.

Clarify the example explaining that .for expansion takes place before
.if handling by showing the correct code instead of saying 'the other
way around'. Change a variable name there so the example is more parseable
to the human reader.

PR: docs/65400
Submitted by: Roman Neuhauser <neuhauser@chello.cz>