Add more detail about how to invoke Eshell commands

Add more detail about how to invoke Eshell commands

* doc/misc/eshell.texi (Variables): Move footnote explaining "REPL"
from here...
(Top): ... to its first use here.
(Commands): Move explanation about kernel functions to here.
(Invocation): Describe command form and Lisp form.  Fix documentation
about priority of commands in command form.
(Arguments): Add a cross reference to the Invocation node.