for Device ZPA GH305 (SML), add a hint for required PIN

First of all: !!! Thanks for your awesome work !!!

@ re PR: arendst/tasmota#

In Tasmota documentation / SmartMeter Interface for Device ZPA GH305 (SML), please add a hint to the (maybe) required PIN to unlock this Device. "Energy provider may have to supply a PIN code to enable output of additional data."

For this device a "PIN" may be required, which can be obtained from the grid provider / meter operator ("Messstellenbetreiber"). See here for example cases where this was necessary: https://github.com/arendst/Tasmota/discussions/18091

Will likely save others from some searching around if they directly get this hint.