Span: Improve documentation
Span: Improve documentation

Makes doxygen reference to SPAN_DYNAMIC_EXTENT by name instead of the macro expansion -1.
Add reference to the macro in the class documentation.

kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os