Made changes to documentation to avoid an error with the upcoming net…

[CarterButts]

This makes a single set of changes to the usage statement for the network.extensions.Rd file, which uses the \method{} tag to trick R into thinking that the *.active functions are S3 methods. We actually had some of these treated this way before, but more have been rendered generic in the meantime.

[skyebend]

@CarterButts when I run cran checks, I see

* checking S3 generic/method consistency ... NOTE
Mismatches for apparent methods not registered:
is.adjacent:
  function(x, vi, vj, na.omit, ...)
is.adjacent.active:
  function(x, vi, vj, onset, terminus, length, at, rule, na.omit,
           active.default)

get.edgeIDs:
  function(x, v, alter, neighborhood, na.omit, ...)
get.edgeIDs.active:
  function(x, v, onset, terminus, length, at, alter, neighborhood,
           rule, na.omit, active.default)

get.edges:
  function(x, v, alter, neighborhood, na.omit, ...)
get.edges.active:
  function(x, v, onset, terminus, length, at, alter, neighborhood,
           rule, na.omit, active.default)

get.neighborhood:
  function(x, v, type, na.omit, ...)
get.neighborhood.active:
  function(x, v, onset, terminus, length, at, type, rule, na.omit,
           active.default)
See section 'Registering S3 methods' in the 'Writing R Extensions'
manual.

which I think is pointing out that ordering of the arguments and "..." don't align, which will probably require a shallow but breaking refactor?