rc.conf(5): Style issues

Fix mandoc -Tlint style issues

• Use Pa instead of Va for .conf element (mdoc warned the Va was unused, and its the wrong markup anyway)
• Drop useless Va and use Va instead of Ar when referring to variables in the jail file
• One sentence, one line
• drop xr to info(1). That makes no sense, and intro(1) seems unhelpful.

This leaves two warnings: gdb(1) xref not found (but we add a
parenthetical about the package) and a false positive about
a trailing period that is being mistaken for a 'full stop'
when it is really just a character in a filename.

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