Document that block splitter doesn't know about padding when reading blocks

[yousefmoazzam]

In the task runner, the max slices is calculated and then reduced by the number of padding slices, before being given to the block splitter:

httomo/httomo/runner/task_runner.py

Lines 106 to 111 in fbfbc20

	slicing_dim_section: Literal[0, 1] = _get_slicing_dim(section.pattern) - 1 # type: ignore
	self.determine_max_slices(section, slicing_dim_section)
	# Account for potential padding in number of max slices
	padding = determine_section_padding(section)
	section.max_slices -= padding[0] + padding[1]

The block splitter in turn will read blocks from its DataSetSource implementor, unaware of the blocks being read will potentially contain padded slices. (Ie, if padding is (2, 2) and the block splitter is told to read a block of length 5, it'll get a block with slicing dim length 5 + 2 + 2 = 9).

It'd be useful to document this more clearly somewhere, before I forget this fact once again...