docs/commandline: Minor formatting fixes and clarifications

docs/commandline: Minor formatting fixes and clarifications

`font` had a trailing single quote which was out of place.

`gnttab_max_frames` was missing escapes for the underscores which caused the
underscores to take their markdown meaning, causing 'max' in the middle to be
italicised.  Escape the underscores, and make all command line parameters
bold, to be consistent with the existing style.

Clarify how the default for `nmi` changes between debug and non debug builds
of Xen.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>