doc: Update LTTng-UST Callstack documentation

doc: Update LTTng-UST Callstack documentation

* Document that the 'vpid' context should now be enabled (it is not
  mandatory, but it's ideal to have it).
* Mention both the "Import Binary" and "Import Mapping File" methods
* Mention that the import step is only necessary for traces taken
  with UST < 2.8.

Change-Id: I2fd76ba41ffd166a76c20ce7929a7b7769974b29
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>
Reviewed-on: https://git.eclipse.org/r/70081
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>