update manpage for numentryguards / numdirectoryguards

[[NumEntryGuards]] **NumEntryGuards** __NUM__::

    If UseEntryGuards is set to 1, we will try to pick a total of NUM routers

    as long-term entries for our circuits. (Default: 3)

    as long-term entries for our circuits. If NUM is 0, we try to learn

    the number from the NumEntryGuards consensus parameter, and default

    to 3 if the consensus parameter isn't set. (Default: 0)

[[NumDirectoryGuards]] **NumDirectoryGuards** __NUM__::

    If UseEntryGuardsAsDirectoryGuards is enabled, we try to make sure we

    have at least NUM routers to use as directory guards. If this option

    is set to 0, use the value from NumEntryGuards. (Default: 0)

    is set to 0, use the value from the NumDirectoryGuards consensus

    parameter, falling back to the value from NumEntryGuards if the

    consensus parameter is 0 or isn't set. (Default: 0)

[[GuardLifetime]] **GuardLifetime**  __N__ **days**|**weeks**|**months**::

    If nonzero, and UseEntryGuards is set, minimum time to keep a guard before