[assembler] Update the summary of unimplemented features.

Author: youngman-g

OPEN-QUESTIONS.md

@@ -88,3 +88,4 @@
    references (e.g. the one at 200003) precede it?  In fact the words
    assembled at 200147 and 200151 also have RC-word references which
    are assigned in the obvious way.
+6. What did M4's ☛☛DEMO meta-command do?

assembler/README.md

@@ -43,12 +43,47 @@ There are some other examples in the `examples` folder.
 
 ## Limitations
 
-The assembler isn't finished yet, so there are a number of quite
-severe limitations:
+### Unimplemented Features
 
-* No support for deferred operands or RC ("Register-Containing") words.
-* No support for macros.  Confusingly, the TX-2 assembler supported
-  macros and was called M4, but is unrelated to the Unix program `m4`.
+Some features are not yet supported, or are supported only a limited
+way:
+
+* No support for macros; these can currently be defined but cannot be
+  used.  Confusingly, the TX-2 assembler supported macros and was
+  called M4, but is unrelated to the Unix program `m4`.
+* The `T` command should control the interpretation of the tab
+  character, but this is not yet supported.
+* The ☛☛RC command is not implemented. It's not clear that
+  implementing it would be useful.
+
+### Redundant Commands
+
+The TX-2 M4 asembler provided some metacommands which aren't useful on
+modern computers having general-purpose text-editing facilities.  The
+☛☛XXX, ☛☛INSERT, ☛☛DELETE and ☛☛REPLACE editing commands aren't needed
+or implemented.
+
+Similarly, the metacommands ☛☛TYPE, ☛☛LIST, ☛☛DIR, ☛☛LDIR, ☛☛TDIR, and
+☛☛PLIST are not implemented.  Instead pass the flag `--list` to the
+assembler to obtain a listing.
+
+The functions of the ☛☛LW, ☛☛READ and ☛☛SAVE metacommands are provided
+by the assembler command-line.  The ☛☛RECONVERT metacommand isn't
+described in any detail in the User Handbook document, but it also
+probably isn't needed.
+
+### Memory I/O
+
+The ☛☛CORE and ☛☛TAPE commands are probably better implemented somehow
+in the simulator than in the assembler.  Similarly for ☛☛GOTO.
+
+The ☛☛BIN and ☛☛CLEAN commands probably aren't needed and so are not
+implemented.
+
+### Undocumented Commands
+
+The Users Handbook mentions the ☛☛DEMO metacommand but doesn't explain
+what it does, so this is not implemented.
 
 ## Documentation