Update the setDtimMultiplier API documentation.

Update the API documentation to describe how setDtimMultiplier API is
supposed to work, i.e. we can call this API to set a maximum DTIM
multiplier value to use in driver, any larger value must not be accepted
and meanwhile it allows to set a specific multiplier by OEMs. Try to
make the description clear and avoid the ambiguity.

Bug: 266256943
Test: m
Change-Id: Iaa65c1acdec94ce6821ff39603b8be50181fe06c
3 files changed