Update and extend the documentation of expm, logm and sqrtm

[mfasi]

Documentation update originally in #12408, moved here after #11943 was merged.

[tkelman]

I think docstrings actually need to change in helpdb.jl now, not in the rst. But please stand by for an announcement on that.

[mfasi]

@tkelman OK, thank you. No problem, I just wanted to avoid having documentation and consistency together, as the latter seems to need some more work. Now that I can keep them separate I'm happy.

[tkelman]

The docs for expm now live here -

julia/base/docs/helpdb.jl

Lines 10748 to 10755 in 7023fc3

	doc"""
	```rst
	::
	expm(A)
	Compute the matrix exponential of ``A``.
	```
	"""

sqrtm and logm should live somewhere else in the file, but I believe it should be safe right now to move the docstrings to the location of those functions' source. You can run ./julia doc/genstdlib.jl to see what the generated rst docs should look like.

[mfasi]

@tkelman Maybe something's not working properly yet. By running ./julia doc/genstdlib.jl I get something like

INFO: no docs for Base.ndims
INFO: no docs for Base.size
INFO: no docs for Base.iseltype
INFO: no docs for Base.length
INFO: no docs for Base.eachindex
INFO: no docs for Base.Base.linearindexing
INFO: no docs for Base.countnz
INFO: no docs for Base.conj!
INFO: no docs for Base.stride
INFO: no docs for Base.strides
INFO: no docs for Base.ind2sub
...

and the .rst files are not changed.

EDIT: it seems that travis can generate the .rst, so it might be a local problem.

[tkelman]

I think those warnings are normal, but the rst files should be changing. Not sure what's up there.

[mfasi]

@tkelman I figured out what I did wrong: each time you change something in base/docs/helpdb.jl, you have to recompile julia before running ./julia doc/genstdlib.jl. Anyway, the generated documentation looks fine, feel free to merge this PR.

[ViralBShah]

I think this deserves a mention in NEWS.