COMP: Inline static DESCRIPTION in itk-module.cmake

Replace the set(DOCUMENTATION "...") + DESCRIPTION "${DOCUMENTATION}"
indirection with a static one-liner literal.

itk_module() is a CMake macro, so ${ARGN} re-tokenizes its arguments
and list-splits any embedded ";". A future edit adding a semicolon or
"[" to the DOCUMENTATION string would silently produce spurious
"Unknown argument" AUTHOR_WARNINGs from CMake/ITKModuleMacros.cmake:111
on every configure (see PRs #6220, #6245).

The v4 ingestion pipeline (PR #6204) enforces this via
sanitize-history.py:patch_dynamic_description.

Author: hjmjohnson

Modules/Filtering/HigherOrderAccurateGradient/itk-module.cmake

@@ -1,14 +1,4 @@
 # Maintainer: Matt McCormick <matt.mccormick@kitware.com>
-set(
-  DOCUMENTATION
-  "This module contains a filter to compute higher order
-accurate numerical derivatives and gradients from an input scalar image.
-Higher Order Accurate Derivative and Gradient Calculation in ITK
-https://www.insight-journal.org/browse/publication/775
-https://hdl.handle.net/10380/3231
-"
-)
-
 itk_module(
   HigherOrderAccurateGradient
   DEPENDS
@@ -19,5 +9,6 @@ itk_module(
   TEST_DEPENDS
     ITKTestKernel
   EXCLUDE_FROM_DEFAULT
-  DESCRIPTION "${DOCUMENTATION}"
+  DESCRIPTION
+    "Filters that compute higher-order accurate numerical derivatives and gradients from a scalar image (Insight Journal: https://www.insight-journal.org/browse/publication/775, https://hdl.handle.net/10380/3231)."
 )