Marc,
Standard practice is to use in the documentation the same term (and
capitalisation) that appears on the item in the user interface. So if
the button says "Login" then the docs would say "You will find the
Login button here." or "Click the Login button." or similar.
As for what the button itself should be called, I'm fairly sure that
the majority of such buttons that I have seen recently are labelled
"Login" or "Logout".
The spelling of "login" and "logout" (with or without a hyphen or
space) in other sentence constructions is different. For example, as a
verb form we would normally write, "To log in, do xxxx." NOT "To
login, do xxxx." In the past, we've written things like, "The log-in
procedure is as follows..." (where log-in is an adjective) but these
days I personally would drop the hyphen and write, "The login
procedure..." just to avoid having too many different spellings, the
nuances of which are totally lost on most readers. I am aware that
some other technical editors have different views on this subject.
Hope this helps and does not further confuse things for the website team.
--Jean