DOC: Explain ElastixRegistrationMethod use "-tp" for initial transforms

[N-Dekker]

Follow-up to pull request #909 commit 9e64269 "ENH: ElastixRegistrationMethod writes initial transform parameter files"

Requested by Marius Staring (@mstaring).

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Main part of the explanation:

elastix/Core/Main/itkElastixRegistrationMethod.hxx

Lines 197 to 200 in b09c835

	// Pass the last initial transform parameter file name to the argument map, in order to have it stored by
	// elx::TransformBase::ReadFromFile(), so that it can be retrieved later by
	// elx::TransformBase::GetInitialTransformParametersFileName(). Use "-tp", instead of "-t0", to avoid actual file
	// reading of the initial transforms, as they are already in memory.