DOC: Warn of lossless float precision change in IO module migration guide

Several IO modules (NIfTI, MetaImage, NRRD, VoxBoCUB, SpatialObject, Mesh)
now use itk::ConvertNumberToString() for float/double metadata, replacing
the 6-significant-digit default. Tests comparing raw header text against
golden baselines may need updating.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hjmjohnson

Documentation/docs/migration_guides/itk_6_migration_guide.md

@@ -213,6 +213,22 @@ The handful of manually wrapped long double functions were
 removed from python wrapping.
 
 
+IO modules write floating-point metadata with lossless precision
+-----------------------------------------------------------------
+
+`float` and `double` metadata values are now serialized using
+`itk::ConvertNumberToString()`, which produces the shortest decimal string
+that round-trips exactly — replacing the previous 6-significant-digit default.
+
+Affected modules: **NIfTI** (`scl_slope`, `scl_inter`, `pixdim`, spacing,
+and offset fields), **MetaImage**, **NRRD**, **VoxBoCUB**, **SpatialObject**,
+and **Mesh IO**.
+
+Tests that compare raw header text against golden baselines may fail because
+values previously written as e.g. `"1"` are now written as `"1.0000001"`.
+Replace exact string matches with numeric comparisons or regenerate baselines.
+Files not written by ITK are unaffected.
+
 Legacy GoogleTest Target Names Removed
 --------------------------------------