* Don't share doctest state between docstrings

This is not, strictly speaking, well defined. There is no official rule
as to which docstring should get executed first.

This will not be supported in a future version of Documenter.

* Add Channel(::Function) to the manual

Also fix the doctest in the docstring. The simpler println variant seems
to be more reliable when printing from a another task (with at-show, the
"Hello" string did not actually get printed).

* Fix Base.splat doctest

The function is not exported.  Note that it does not currently get
doctested, but will be tested in a future Documenter version.

* Fix doctest in at-locals docstring

This is not currently included in the manual and therefore not
doctested, but will be in the future.

* Fix doctest in Base.isambiguous docstring

Not currently doctested since it is not included in the manual, but will
be in the future.

* Add findnext/last(::AbstractChar,...) to manual

Also fix a doctest in one of the docstrings.

* Add BLAS.dot and axpby! to manual

And fix the doctests in the docstrings.

* Fix doctests of unexported functions

Fix doctests in the docstrings of LinearAlgebra.isbanded, 
LinearAlgebra.promote_leaf_eltypes and SparseArrays.dropstored!. All the 
functions are unexported and not included in the manual.

They are currently not being doctested, but will be with a future 
version of Documenter.