docs: add identifiers (#11174)

* docs: add identifiers

* clarify attribute set notation and add examples

* add definition of names

Co-authored-by: Ryan Hendrickson <ryan.hendrickson@alum.mit.edu>

doc/manual/src/language/identifiers.md

@@ -0,0 +1,50 @@
+# Identifiers
+
+An *identifier* is an [ASCII](https://en.wikipedia.org/wiki/ASCII) character sequence that:
+- Starts with a letter (`a-z`, `A-Z`) or underscore (`_`)
+- Can contain any number of:
+  - Letters (`a-z`, `A-Z`)
+  - Digits (`0-9`)
+  - Underscores (`_`)
+  - Apostrophes (`'`)
+  - Hyphens (`-`)
+- Is not one of the [keywords](#keywords)
+
+> **Syntax**
+>
+> *identifier* ~ `[A-Za-z_][A-Za-z0-9_'-]*`
+
+# Names
+
+A name can be an [identifier](#identifier) or a [string literal](./syntax.md#string-literal).
+
+> **Syntax**
+>
+> *name* → *identifier* | *string*
+
+Names are used in [attribute sets](./syntax.md#attrs-literal), [`let` bindings](./syntax.md#let-expressions), and [`inherit`](./syntax.md#inheriting attributes).
+
+# Keywords
+
+These keywords are reserved and cannot be used as [identifiers](#identifiers):
+
+- [`assert`](./syntax.md#assertions)
+- [`else`][if]
+- [`if`][if]
+- [`in`][let]
+- [`inherit`](./syntax.md#inheriting-attributes)
+- [`let`][let]
+- [`or`](./operators.md#attribute-selection) (see note)
+- [`rec`](./syntax.md#recursive-sets)
+- [`then`][if]
+- [`with`](./syntax.md#with-expressions)
+
+[if]: ./syntax.md#conditionals
+[let]: ./syntax.md#let-expressions
+
+> **Note**
+>
+> The Nix language evaluator currently allows `or` to be used as a name in some contexts, for backwards compatibility reasons.
+> Users are advised not to rely on this.
+>
+> There are long-standing issues with how `or` is parsed as a name, which can't be resolved without making a breaking change to the language.