dt-bindings: battery: Clarify trickle-charge

The term 'trickle-charging' is used to describe a very slow charging
phase, where electrons "trickle-in" the battery.

There are two different use-cases for this type of charging. At least
some Li-Ion batteries can benefit from very slow, constant current,
pre-pre phase 'trickle-charging', if a battery is very empty.

Some other batteries use top-off phase 'trickle-charging', which is
different from the above case.

The battery bindings use the term 'trickle-charge' without specifying
which of the use-cases properties are addressing. This has already
caused some confusion.

Clarify that the 'trickle-charge-current-microamp' refers to the first
one, the "pre-pre" -charging use-case.

Suggested-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patch.msgid.link/e2794140343103245410c3301f8994e1babaeb96.1765804226.git.mazziesaccount@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>