Document numeric type literals in lexical conventions. (#4842)

Also improve the precision of some other nearby documentation.

Author: zygoloid

docs/design/expressions/literals.md

@@ -14,7 +14,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 -   [Numeric literals](#numeric-literals)
     -   [Numeric literal syntax](#numeric-literal-syntax)
-    -   [Defined Types](#defined-types)
+    -   [Defined types](#defined-types)
     -   [Implicit conversions](#implicit-conversions)
     -   [Examples](#examples)
     -   [Alternatives Considered](#alternatives-considered)
@@ -70,7 +70,7 @@ Numeric literal syntax is covered in the
 doc. Both Integer and Real-Number syntax is defined, with decimal, hexadecimal
 and binary integer literals, and decimal and hexadecimal real number literals.
 
-### Defined Types
+### Defined types
 
 The following types are defined in the Carbon prelude:
 

docs/design/lexical_conventions/words.md

@@ -12,6 +12,8 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 -   [Overview](#overview)
 -   [Keywords](#keywords)
+    -   [Type literals](#type-literals)
+    -   [Identifiers](#identifiers)
     -   [Raw identifiers](#raw-identifiers)
 -   [Alternatives considered](#alternatives-considered)
 -   [References](#references)
@@ -21,7 +23,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 ## Overview
 
 A _word_ is a lexical element formed from a sequence of letters or letter-like
-characters, such as `fn` or `Foo` or `Int`.
+characters, such as `fn` or `Foo` or `Int`, optionally preceded by `r#`.
 
 The exact lexical form of words has not yet been settled. However, Carbon will
 follow lexical conventions for identifiers based on
@@ -98,20 +100,47 @@ The following words are interpreted as keywords:
 -   `where`
 -   `while`
 
+### Type literals
+
+A word starting with `i`, `u`, or `f`, followed by a decimal integer, is a
+[_numeric type literal_](/docs/design/expressions/literals.md#numeric-type-literals).
+
+### Identifiers
+
+A word is interpreted as an _identifier_ if it is neither a keyword nor a type
+literal.
+
 ### 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.
+A _raw identifier_ is a word starting with `r#`. A raw identifier is equivalent
+to the word following the `r#` prefix, except that it is always interpreted as
+an identifier, even if it would otherwise be a keyword or type literal.
+
+Raw identifiers can be used to specify identifiers which have the same spelling
+as keywords; for example, `r#impl`. This can be useful when interoperating with
+C++ code that uses identifiers that are keywords in Carbon, and when migrating
+between versions of 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>`.
+The word doesn't need to be a keyword, in order to support forwards
+compatibility when a keyword is planned to be added. If `word` is an identifier,
+then `word` and `r#word` have the same meaning.
 
 ## Alternatives considered
 
+Overview:
+
 -   [Character encoding: We could restrict words to ASCII.](/proposals/p0142.md#character-encoding-1)
 -   [Normalization form alternatives considered](/proposals/p0142.md#normalization-forms)
+
+Type literals:
+
+-   [Use C++ type keywords with LP64 convention](/proposals/p2015.md#c-lp64-convention)
+-   [Use full type name with length suffix](/proposals/p2015.md#type-name-with-length-suffix)
+-   [Use uppercase for type names](/proposals/p2015.md#uppercase-suffixes)
+-   [Support additional bit widths](/proposals/p2015.md#additional-bit-sizes)
+
+Raw identifiers:
+
 -   [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)
@@ -121,5 +150,7 @@ a keyword, it will refer to the same entity as `r#<identifier>`.
 
 -   Proposal
     [#142: Unicode source files](https://github.com/carbon-language/carbon-lang/pull/142)
+-   Proposal
+    [#2015: Numeric type literal syntax](https://github.com/carbon-language/carbon-lang/pull/2015)
 -   Proposal
     [#3797: Raw identifier syntax](https://github.com/carbon-language/carbon-lang/pull/3797)