[lldb] Add release note for "register info" command

llvm/docs/ReleaseNotes.rst

* LLDB is now able to show the subtype of signals found in a core file. For example
   memory tagging specific segfaults such as ``SIGSEGV: sync tag check fault``.
 
 * LLDB can now display register fields if they are described in target XML sent
   by a debug server such as ``gdbserver`` (``lldb-server`` does not currently produce
   this information). Fields are only printed when reading named registers, for
   example ``register read cpsr``. They are not shown when reading a register set,
   ``register read -s 0``.
 
+* A new command ``register info`` was added. This command will tell you everything that
+  LLDB knows about a register. Based on what LLDB already knows and what the debug
+  server tells it. Including but not limited to, the size, where it is read from and
+  the fields that the register contains.
+
 Changes to Sanitizers
 ---------------------
 * For Darwin users that override weak symbols, note that the dynamic linker will
   only consider symbols in other mach-o modules which themselves contain at
   least one weak symbol. A consequence is that if your program or dylib contains
   an intended override of a weak symbol, then it must contain at least one weak
   symbol as well for the override to take effect.