Add RELEASE.txt note for the fix for issue #1256 (#3955)

release_docs/RELEASE.txt

@@ -476,6 +476,34 @@ Bug Fixes since HDF5-1.14.0 release
 ===================================
     Library
     -------
+    - Memory usage growth issue
+
+      Starting with the HDF5 1.12.1 release, an issue (GitHub issue #1256)
+      was observed where running a simple program that has a loop of opening
+      a file, reading from an object with a variable-length datatype and
+      then closing the file would result in the process fairly quickly
+      running out of memory. Upon further investigation, it was determined
+      that this memory was being kept around in the library's datatype
+      conversion pathway cache that is used to speed up datatype conversions
+      which are repeatedly used within an HDF5 application's lifecycle. For
+      conversions involving variable-length or reference datatypes, each of
+      these cached pathway entries keeps a reference to its associated file
+      for later use. Since the file was being closed and reopened on each
+      loop iteration, and since the library compares for equality between
+      instances of opened files (rather than equality of the actual files)
+      when determining if it can reuse a cached conversion pathway, it was
+      determining that no cached conversion pathways could be reused and was
+      creating a new cache entry on each loop iteration during I/O. This
+      would lead to constant growth of that cache and the memory it consumed,
+      as well as constant growth of the memory consumed by each cached entry
+      for the reference to its associated file.
+
+      To fix this issue, the library now removes any cached datatype
+      conversion path entries for variable-length or reference datatypes
+      associated with a particular file when that file is closed.
+
+      Fixes GitHub #1256
+
     - Suppressed floating-point exceptions in H5T init code
 
       The floating-point datatype initialization code in H5Tinit_float.c