Help: Add documentation on escaping changes with CMP0053

Help/manual/cmake-language.7.rst

@@ -296,16 +296,18 @@ An *escape sequence* is a ``\`` followed by one character:
 
 .. productionlist::
  escape_sequence: `escape_identity` | `escape_encoded` | `escape_semicolon`
- escape_identity: '\(' | '\)' | '\#' | '\"' | '\ ' |
-                : '\\' | '\$' | '\@' | '\^'
+ escape_identity: '\' <match '[^A-Za-z0-9;]'>
  escape_encoded: '\t' | '\r' | '\n'
  escape_semicolon: '\;'
 
-A ``\`` followed by one of ``()#" \#@^`` simply encodes the literal
+A ``\`` followed by a non-alphanumeric character simply encodes the literal
 character without interpreting it as syntax.  A ``\t``, ``\r``, or ``\n``
-encodes a tab, carriage return, or newline character, respectively.
-A ``\;`` encodes itself but may be used in an `Unquoted Argument`_
-to encode the ``;`` without dividing the argument value on it.
+encodes a tab, carriage return, or newline character, respectively. A ``\;``
+outside of any `Variable References`_  encodes itself but may be used in an
+`Unquoted Argument`_ to encode the ``;`` without dividing the argument
+value on it.  A ``\;`` inside `Variable References`_ encodes the literal
+``;`` character.  (See also policy :policy:`CMP0053` documentation for
+historical considerations.)
 
 .. _`Variable References`:
 
@@ -319,6 +321,11 @@ or by the empty string if the variable is not set.
 Variable references can nest and are evaluated from the
 inside out, e.g. ``${outer_${inner_variable}_variable}``.
 
+Literal variable references may consist of alphanumeric characters,
+the characters ``/_.+-``, and `Escape Sequences`_.  Nested references
+may be used to evaluate variables of any name.  (See also policy
+:policy:`CMP0053` documentation for historical considerations.)
+
 The `Variables`_ section documents the scope of variable names
 and how their values are set.