DOC: Update FFTPadImageFilter prime factor doc for PocketFFT

[blowekamp]

Add PocketFFT (11) to the SizeGreatestPrimeFactor default-value list in FFTPadImageFilter; fix minor typo ("values is" → "value is").

AI assistance

GitHub Copilot located the PocketFFT GetSizeGreatestPrimeFactor return value (11) and drafted the comment update.

[blowekamp]

The read me for pocket fft has the following:

Efficient codelets are available for the factors:

2, 3, 4, 5, 7, 8, 11 for complex-valued FFTs
2, 3, 4, 5 for real-valued FFTs

There was also some uncertainty if the FFTW value was correct too. This doc says it is only optimized up to 7: https://www.fftw.org/fftw2_doc/fftw_1.html

[hjmjohnson]

One correction and one adjacent cleanup, both as one-click suggestions. (Supersedes my earlier approval.)

Verified values from the code on main

• FFTWProxyType::GREATEST_PRIME_FACTOR is 13 (7 when built with ITK_USE_CUFFTW) — itkFFTWCommon.h. The fftw2 doc reference predates FFTW3's codelet set; ITK's constant is what this doc should describe.
• All PocketFFT filters (including the real-valued RealToHalfHermitian/HalfHermitianToReal variants) return 11 from GetSizeGreatestPrimeFactor(). If the real-valued paths should return 5 per the pocketfft README, that is a code change to make first — this doc should match what the code returns today.
• The VNL/Temperton FFT backend was removed; the Vnl*FFTImageFilter names are now deprecated aliases deriving from the PocketFFT filters. They do not override GetSizeGreatestPrimeFactor(), so they inherit and report 11, never 5.

[hjmjohnson]

The VNL/Temperton backend was removed; the deprecated Vnl* aliases derive from the PocketFFT filters and inherit 11, so "5 for VNL" is no longer reachable.

Suggested change

	* obtained from the ForwardFFTImageFilter, which is 13 for FFTW, 11 for PocketFFT,
	* and 5 for VNL.
	* obtained from the ForwardFFTImageFilter, which is 13 for FFTW (7 with cuFFTW)
	* and 11 for PocketFFT.

[hjmjohnson]

Grammar, and the next line's "is suitable for vnl base fft filters." is also stale (VNL backend removed) — it is just outside this hunk, so please fold it in manually, e.g. end the sentence at "power of 2." and drop the VNL clause.

Suggested change

	* A greatest prime factor of 2 produce a size which is a power of 2, and thus
	* A greatest prime factor of 2 produces a size which is a power of 2, and thus