rigol-ds: add a few more DS2000 models to the supported list
rigol-ds: fix waveform reception on DS2000 series

The ":WAV:DATA?" scpi command must be sent before calling
sr_scpi_read_begin().
scpi_usbtmc: fix reading of blocks bigger than the 2048 bytes buffer
serial: Improved docs.
Merge branch 'master' of git://sigrok.org/libsigrok