Typically the instruction booklet provided is universal (i.e. covering all M&P15s in general rather than any specific model) and mainly covers safety as well as field-stripping instructions rather than details on how to completely disassemble the rifle.
Alas, I am unfamiliar with how to remove the handguard, but it ought to be in alignment with the toprail on the receiver so that front and rear sights line up properly.
I searched around and found the following thread:
M&P 15T quad-rail removal
Granted this is for the original M&P15T, but hopefully it will help.