Describe passing custom arguments to snippets

This commit adds description how to pass field or custom argument values
to snippets using some small code examples.

caused by https://forum.sublimetext.com/t/how-do-i-insert-a-snippet-with-paramn-variables-from-a-plugin/76200

Author: deathaxe

docs/guide/extensibility/snippets.md

@@ -98,13 +98,12 @@ element's text in examples unless otherwise noted.
 ### Environment Variables
 
 Snippets have access to contextual information in the form of
-environment variables. The values of the variables listed below are set
+environment variables. The values of variables listed below are set
 automatically by Sublime Text.
 
 
 |      Variable        |                           Description                                 |
 | -------------------- | --------------------------------------------------------------------- |
-| `$PARAM1 .. $PARAMn` | Arguments passed to the `insert_snippet` command. (Not covered here.) |
 | `$SELECTION`         | The text that was selected when the snippet was triggered.            |
 | `$TM_CURRENT_LINE`   | Content of the cursor's line when the snippet was triggered.          |
 | `$TM_CURRENT_WORD`   | Word under the cursor when the snippet was triggered.                 |
@@ -295,3 +294,90 @@ Transformation: ${TM_FILENAME/(\w+)\.js/\1/g}
 
 Transformation: MyModule
 ```
+
+### Passing custom arguments
+
+Command Palette items, key bindings and plugins
+can pass values 
+of fields (`$1` or `${1:placeholder}`)
+or custom arguments (`$any_variable`)
+to snippets,
+when inserting them via [insert_snippet][] command.
+
+The following examples insert `"Hello World!"` to buffer.
+
+Let's assume a generic snippet _Packages/User/My Snippet.sublime-snippet_
+which provides a field `$1` 
+and expects a custom argument `$subject`:
+
+```xml
+<snippet>
+   <content><![CDATA[$1 $subject!]]></content>
+   <description>Snippet with 2 fields</description>
+</snippet>
+```
+
+It can be triggered by key bindings with static values being passed.
+
+```json
+[
+  { 
+    "keys": ["ctrl+k", "ctrl+w"],
+    "command": "insert_snippet",
+    "args": {
+      "name": "Packages/User/My Snippet.sublime-snippet",
+      "1": "Hello",
+      "subject": "World"
+    }
+  }
+]
+```
+
+Plugins can pass dynamically determined values.
+
+```py
+import sublime_plugin
+
+class MyInsertSnippetCommand(sublime_plugin.TextCommand):
+  def run(self, edit):
+    field1 = "Hello"
+    
+    self.view.run_command(
+      cmd="insert_snippet",
+      args={
+        "name": "Packages/User/My Snippet.sublime-snippet",
+        "1": field1,
+        "subject": self.get_subject()
+      }
+    )
+
+  def get_subject(self):
+    return "World"
+```
+
+Field values and arguments can also be passed to inline snippet content.
+
+```py
+import sublime_plugin
+
+class MyInsertSnippetCommand(sublime_plugin.TextCommand):
+  def run(self, edit):
+
+    snippet_content = "$1 $subject!"
+
+    field1 = "Hello"
+    
+    self.view.run_command(
+      cmd="insert_snippet",
+      args={
+        "contents": snippet_content,
+        "1": field1,
+        "subject": self.get_subject()
+      }
+    )
+
+  def get_subject(self):
+    return "World"
+```
+
+[insert_snippet]: ../../reference/commands#insert_snippet

docs/reference/commands.md

@@ -331,6 +331,8 @@ Inserts a snippet from a string or *.sublime-snippet* file.
   backslashes `\` have to be escaped, like in every other JSON string.
 - **name** (String): [Relative path][About Paths in Command Arguments] to the
   *.sublime-snippet* file to be inserted.
+- **args** (String): a list of key-value pairs (e.g.: `"myparam": "value"`)
+  specifying values to replace arbitrary variables (e.g.: `$myparam`) with.
 
 ::: info See Also
 [Snippets][]