Clarify the purpose of _STRING_LEX
Clarify the purpose of _STRING_LEX

The main purpose of _STRING_LEX is being able to tell strings from
constant symbol references during tokenization. The old comment was
implying that its only purpose is handling the "missing quotes" in e.g.

    source foo/Kconfig

Update the comment to make _STRING_LEX clearer.

ulfalizer/Kconfiglib,ulfalizer/Kconfiglib