Cherry-pick fixes from bugfix/10104_conjoinworkspaces_fixspectrum

into next

Changes:

    Refs #10104. Fixed the issue.

    and added some sentences in docs.
    (cherry picked from commit acd0cc879d1095aa9b049ea06d313db0eb222b12)

    Conflicts:

	Code/Mantid/docs/source/algorithms/ConjoinWorkspaces-v1.rst