Doxygen: Fix various warnings.

Doxygen: Fix various warnings.

  src/resource.c:414: warning: unbalanced grouping commands

  conversion.c:81: warning: argument 'lo_thr' from the argument list of sr_a2l_schmitt_trigger has multiple @param documentation sections

  src/analog.c:611: warning: return value 'SR_ERR_ARG' of sr_rational_div has multiple documentation sections

  src/device.c:205: warning: explicit link request to 'TRUE' could not be resolved
  src/device.c:205: warning: explicit link request to 'FALSE' could not be resolved
  src/device.c:231: warning: explicit link request to 'TRUE' could not be resolved
  src/device.c:231: warning: explicit link request to 'FALSE' could not be resolved

  src/serial.c:246: warning: explicit link request to 'NULL' could not be resolved

  src/strutil.c:602: warning: explicit link request to 'NULL' could not be resolved

  src/device.c:94: warning: unable to resolve reference to 'sr_channel_free()' for \ref command

  src/strutil.c:597: warning: unable to resolve reference to 'sr_hexdump_free()' for \ref command
  src/strutil.c:622: warning: unable to resolve reference to 'sr_hexdump_new()' for \ref command

  src/device.c:430: warning: The following parameters of sr_dev_inst_channel_add(struct sr_dev_inst *sdi, int index, int type, const char *name) are not documented: parameter 'sdi'

  src/session.c:163: warning: The following parameters of fd_source_new(struct sr_session *session, void *key, gintptr fd, int events, int timeout_ms) are not documented: parameter 'events'