Miscellaneous docstring additions and fixes

[Saransh-cpp]

Added missing docstrings (in the manual), updated the existing ones, and added doctests in the following files -

• stateless.jl
• losses/functions.jl
• utils.jl

In addition, there are various miscellaneous docstring fixes, and I have also prepended an _ to the internal API. This PR should close #1990.

This PR also closes #1993, but I am not 100% sure if the current implementation is wrong. I will revert the changes back if everything is correct.

PR Checklist

• [ ] Tests are added
• [ ] Entry in NEWS.md
• [ ] Documentation, if applicable

[codecov-commenter]

Codecov Report

Merging #1998 (de72a12) into master (0b01b77) will not change coverage. The diff coverage is 87.50%.

@@           Coverage Diff           @@
##           master    #1998   +/-   ##
=======================================
  Coverage   87.10%   87.10%           
=======================================
  Files          20       20           
  Lines        1528     1528           
=======================================
  Hits         1331     1331           
  Misses        197      197           

Impacted Files	Coverage Δ
src/layers/stateless.jl	100.00% <ø> (ø)
src/losses/ctc.jl	93.81% <ø> (ø)
src/layers/normalise.jl	88.81% <60.00%> (ø)
src/utils.jl	95.30% <90.00%> (ø)
src/layers/basic.jl	77.32% <100.00%> (ø)
src/layers/conv.jl	88.00% <100.00%> (ø)
src/losses/functions.jl	98.64% <100.00%> (ø)

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[Saransh-cpp]

cc: @DhairyaLGandhi

[Saransh-cpp]

I have renamed rng_from_array() (with no arguments) to default_rng_value(), and added both of them to the manual.

[ToucheSir]

Thanks @Saransh-cpp !