add documentation for language injection

book/src/SUMMARY.md

@@ -16,3 +16,4 @@
   - [Adding Languages](./guides/adding_languages.md)
   - [Adding Textobject Queries](./guides/textobject.md)
   - [Adding Indent Queries](./guides/indent.md)
+  - [Adding Injection Queries](./guides/injection.md)

book/src/guides/injection.md

@@ -0,0 +1,57 @@
+# Adding Injection Queries
+
+Writing language injection queries allows one to highlight a specific node as a different language.
+In addition to the [standard](upstream-docs) language injection options used by tree-sitter, there
+are a few Helix specific extensions that allow for more control.
+
+And example of a simple query that would highligh all strings as bash in Nix:
+```scm
+((string_expression (string_fragment) @injection.content)
+  (#set! injection.language "bash"))
+```
+
+## Capture Types
+
+- `@injection.language` (standard):
+The captured node may contain the language name used to highlight the node captured by
+`@injection.content`.
+
+- `@injection.content` (standard):
+Marks the content to be highlighted as the language captured with `@injection.language` _et al_.
+
+- `@injection.filename` (extension):
+The captured node may contain a filename with a file-extension known to Helix,
+highlighting `@injection.content` as that language. This uses the language extensions defined in
+both the default languages.toml destributed with Helix, as well as user defined languages.
+
+- `@injection.shebang` (extension):
+The captured node may contain a shebang used to choose a language to highlight as. This also uses
+the shebangs defined in the default and user languages.toml.
+
+## Settings
+
+- `injection.combined` (standard):
+Indicates that all of the matching nodes in the tree should have their content parsed as one
+nested document.
+
+- `injection.language` (standard):
+Forces the captured content to be highlighted as the given language
+
+- `injection.include-children`  (standard):
+Indicates that the content node’s entire text should be re-parsed, including the text of its child
+nodes. By default, child nodes’ text will be excluded from the injected document.
+
+- `injection.include-unnamed-children`  (extension):
+Same as `injection.include-children` but only for unamed child nodes.
+
+## Predicates
+
+- `#eq?` (standard):
+The first argument (a capture) must be equal to the second argument
+(a capture or a string).
+
+- `#match?` (standard):
+The first argument (a capture) must match the regex given in the
+second argument (a string).
+
+[upstream-docs]: http://tree-sitter.github.io/tree-sitter/syntax-highlighting#language-injection