gpio: Clarify consumer stubs use-cases

After discussion we add a few blurbs to clarify how the stubs
in the consumer API are supposed to be used.

Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>