Char datatype documentation (signed vs unsigned)

I have been advised in this forum discussion that the char datatype is not always 'signed', and in particular on the arduino zero is treated as 'unsigned' by default.

I suggest thispage should be updated to reflect this because it currently suggests the approach is consistent across all arduino platforms. Thanks

I submitted a pull request to fix this documentation issue: https://github.com/arduino/reference-en/pull/527