Update documentation to reflect that block-end != basic block end

[xabiugarte]

As commented in PR #36, the block end event does not represent necessarily the end of a basic block.

As commented by f4rnham:

"""[...] issues with basic block ends being only subset of translation block ends.
Under certain conditions, like too long basic block or popf, sti, invlpg instructions, TB translation is terminated immediately even though basic block didn't end."""

Instead of dealing with all these individual cases, the documentation should be updated to reflect that block-begin and block-end refer to QEMU translation blocks and may not match basic-block boundaries.

[xabiugarte]

An explanation has been added to the documentation.