Improve documentation of `upload_protocol`

[fpedd]

The default upload_protocol for most of the boards is dslite. Thus, when, for example, looking at this one would assume that mspdebug is an alternative tool. And while this is technically correct, specifying upload_protocol = mspdebug will hand over mspdebug as the first argument to mspdebug, resulting in a call like this mspdebug mspdebug, which in turn obviously results in an error Unknown driver: mspdebug. Try --help for a list. One instead has to notice, that the upload_protocol flag is actually used for one of the drivers that mspdebug supports. So one of these:

Available drivers are:
    rf2500
        eZ430-RF2500 devices. Only USB connection is supported.
    olimex
        Olimex MSP-JTAG-TINY.
    olimex-v1
        Olimex MSP-JTAG-TINY (V1).
    olimex-iso
        Olimex MSP-JTAG-ISO.
    olimex-iso-mk2
        Olimex MSP430-JTAG-ISO-MK2.
    sim
        Simulation mode (standard CPU)
    simx
        CPUX Simulation mode
    uif
        TI FET430UIF and compatible devices (e.g. eZ430).
    uif-bsl
        TI FET430UIF bootloader.
    flash-bsl
        TI generic flash-based bootloader via RS-232
    gdbc
        GDB client mode
    tilib
        TI MSP430 library
    goodfet
        GoodFET MSP430 JTAG
    pif
        Parallel Port JTAG
    gpio
        /sys/class/gpio direct connect
    load-bsl
        Loadable USB BSL driver (USB 5xx/6xx).
    ezfet
        Texas Instruments eZ-FET
    rom-bsl
        ROM bootstrap loader
    bus-pirate
        Bus Pirate JTAG, MISO-TDO, MOSI-TDI, CS-TMS, AUX-RESET, CLK-TCK
    mehfet
        MehFET USB JTAG/SBW device

The two most important ones are rf2500 (for MSP430Gxxx) and tilib (for the rest).

I would suggest documenting that somewhere if it is not already done.