DOC: configuration: explain quotes and spaces in conditional blocks

DOC: configuration: explain quotes and spaces in conditional blocks

Conditional blocks inherit the same tokenizer and argument parser as
the rest of the configuration, but are also silently concatenated
around groups of spaces and tabs. This can lead to subtle failures
for configs containing spaces around commas and parenthesis, where
a string comparison might silently fail for example. Let's better
document this particular case.

Thanks to Valentine for analysing and reporting the problem.

This can be backported to 2.4.