Clean up unit documentation

[n01r]

@SchroederSa brought up that we can still improve the documentation of our units:

Examples from the docs:

• beam.sigmaPx (float, in momentum) rms Px
• beam.sigmaPy (float, in momentum) rms Py

'in momentum' could be more precise

beam.sigmaT (float, in radian) rms normalized time difference T

How can the longitudinal position be in radians?

I assume

beam.sigmaPx (float, in momentum) rms Px ----> beam.sigmaPx (float, in radian) rms Px normalised to P_ref
beam.sigmaT (float, in radian) rms normalized time difference T ----> beam.sigmaT (float, in meter) rms time or similar

beam.sigmaPt (float, in energy deviation) rms Pt

I would consistently write the unit in parentheses and afterwards the description

meaning:
beam.VARIABLE (type, in UNIT) what; how

beam.sigmaPx(float, in radian) rms of x-momentum; normalised to Pz of reference particle

Thanks for bringing this up, @SchroederSa, I'll put it on our agenda. :)