help_topic_target parses the expression being passed to the
help function call to find the innermost module subset expression in
it.
call_help invokes a help() call expression for a package help
topic, finding the first help function definition, ignoring the one
from this package.
help_topic_target(caller, topic)call_help(call, caller)
the environment from which help was called.
the unevaluated expression passed to help.
the patched help call expression.
help_topic_target returns a list of two elements containing
the innermost module of the help call, as well as the name of the
object that<U+2019>s the subject of the help call. For help(a$b$c$d),
it returns list(c, quote(d)).