Add clarifying comment in KeyboardLayout.h

[edgar-bonet]

Users are encouraged to write their own national keyboard layouts, and the comments in KeyboardLayout.h provide the documentation for doing so. It has been suggested to me that it would be good to make crystal clear in this documentation that characters outside the 7-bit ASCII range are not supported.

This pull requests adds this clarification.

[github-actions[bot]]

Memory usage change @ a31b491d18ebcb8a9af6f83a7d52196dbca6e4d9

Board	flash	%	RAM for global variables	%
arduino:avr:leonardo	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:sam:arduino_due_x_dbg	0 - 0	0.0 - 0.0	N/A	N/A
arduino:samd:mkrzero	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board|`examples/Serial`
flash|%|`examples/Serial`
RAM for global variables|% -|-|-|-|- `arduino:avr:leonardo`|0|0.0|0|0.0 `arduino:sam:arduino_due_x_dbg`|0|0.0|N/A|N/A `arduino:samd:mkrzero`|0|0.0|0|0.0

Click for full report CSV

``` Board,examples/Serial
flash,%,examples/Serial
RAM for global variables,% arduino:avr:leonardo,0,0.0,0,0.0 arduino:sam:arduino_due_x_dbg,0,0.0,N/A,N/A arduino:samd:mkrzero,0,0.0,0,0.0 ```

[oqibidipo]

Typo: "Non-ACSII".

[edgar-bonet]

@oqibidipo: Oops! By bad. :worried: Not sure it is worth making another PR to fix it though…