doc: standardize config option names to use interstitial underscores

doc: standardize config option names to use interstitial underscores

Ceph config option names may use spaces, underscores, or by one reference hyphens
as interstitial separators. Most usage within the doc tree uses underscores,
though example conf files and especially structured lists of options mostly
use spaces. Mostly. Underscores help differentiate the config names from
surrounding text, and moreover facilitate scripting, grep, awk, etc and match
their form in src/common/options.cc.

This PR conforms these occurrences of option names to use interstitial underscores instead of spaces.

Fixes: https://tracker.ceph.com/issues/48301
Signed-off-by: Anthony D'Atri <anthony.datri@gmail.com>