doc: camelCase the config variables to improve readability

doc: camelCase the config variables to improve readability

References to multi-word configuration variable names in our
documentation must consistently use camelCase to highlight where
the word boundaries are, even though these are treated case
insensitively.

Fix a few places that spell them in all lowercase, which makes
them harder to read.

Signed-off-by: Kaartic Sivaraam <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>