Add docs for raw identifier syntax. (#4223)

Feature approved in #3797

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

docs/design/lexical_conventions/words.md

@@ -12,6 +12,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 -   [Overview](#overview)
 -   [Keywords](#keywords)
+    -   [Raw identifiers](#raw-identifiers)
 -   [Alternatives considered](#alternatives-considered)
 -   [References](#references)
 
@@ -97,12 +98,28 @@ The following words are interpreted as keywords:
 -   `where`
 -   `while`
 
+### Raw identifiers
+
+A raw identifier looks like `r#<identifier>`. This can be used for identifiers
+which have the same spelling as keywords; for example, `r#impl`. It can help
+when using C++ code with identifiers that are keywords in Carbon.
+
+The identifier doesn't need to be a keyword, in order to support forwards
+compatibility when a keyword is planned to be added. When `<identifier>` is not
+a keyword, it will refer to the same entity as `r#<identifier>`.
+
 ## Alternatives considered
 
 -   [Character encoding: We could restrict words to ASCII.](/proposals/p0142.md#character-encoding-1)
 -   [Normalization form alternatives considered](/proposals/p0142.md#normalization-forms)
+-   [Other raw identifier syntaxes](/proposals/p3797.md#other-raw-identifier-syntaxes)
+-   [Restrict raw identifier syntax to current and future keywords](/proposals/p3797.md#restrict-raw-identifier-syntax-to-current-and-future-keywords)
+-   [Don't require syntax for references to raw identifiers](/proposals/p3797.md#dont-require-syntax-for-references-to-raw-identifiers)
+-   [Don't provide raw identifier syntax](/proposals/p3797.md#dont-provide-raw-identifier-syntax)
 
 ## References
 
 -   Proposal
     [#142: Unicode source files](https://github.com/carbon-language/carbon-lang/pull/142)
+-   Proposal
+    [#3797: Raw identifier syntax](https://github.com/carbon-language/carbon-lang/pull/3797)