Qmk_firmware: LM(layer,mod) documentation

[noleti]

It took me a bit to get LM(layer, mod) to work because I didn't see in the documentation what values are expected for 'mod'. I expected it to be KC_LSFT, for example, which compiled fine but didn't work. After some digging through the sources I found that mods_bit is meant, and the code for LSFT is 2. Maybe this could be improved in the documentation.

[danburkhol]

Until documentation is updated, those experiencing this issue can find the mod codes in
./tmk_core/common/action_code.h