Documentation/config.txt: have a separate "Values" section

Documentation/config.txt: have a separate "Values" section

The various types of values set to the configuration variables
deserve more than a brief footnote mention in the syntax section,
and it will be more so after the later steps of this clean up
effort.

Move the mention of booleans from the syntax section to this new
section, and describe how human-readble integers can be spelled with
scaling there.

Signed-off-by: Junio C Hamano <redacted>