Updated pymc.GaussianRandomWalk docstring

[GallowayJ]

I made the following changes to the docstrings in the GaussianRandomWalk class and its methods:

• Added spaces preceding colons
• Changed occurrences of tensor to TensorVariable
• In class parameters, added optional to type hints
• Added backticks to mentions of variables in parameter class description

These changes are part of work towards issue #5459.

• right before it's ready to merge, mention the PR in the RELEASE-NOTES.md

#DataUmbrellaPyMCSprint

[canyon289]

Thank you for the contribution!

[codecov]

Codecov Report

Merging #5492 (13649d0) into main (af14bea) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5492      +/-   ##
==========================================
+ Coverage   81.44%   81.47%   +0.02%     
==========================================
  Files          81       81              
  Lines       14204    14204              
==========================================
+ Hits        11569    11572       +3     
+ Misses       2635     2632       -3     

Impacted Files	Coverage Δ
pymc/distributions/timeseries.py	22.35% <ø> (ø)
pymc/data.py	80.67% <0.00%> (ø)
pymc/sampling_jax.py	98.30% <0.00%> (ø)
pymc/distributions/continuous.py	97.23% <0.00%> (ø)
pymc/parallel_sampling.py	87.70% <0.00%> (+0.99%)	⬆️

[OriolAbril]

Thanks for the PR!