Document HeaderInsertion config option (#129)

Author: MythreyaK

config.md

@@ -371,10 +371,19 @@ Possible values: `None` (default), `Strict`.
 
 ## Completion
 
+Configures code completion features. Sample block (default):
+
+```yaml
+Completion:
+    AllScopes: Yes
+    ArgumentLists: FullPlaceholders
+    HeaderInsertion: IWYU
+```
+
 ### AllScopes
 {:.v13}
 Whether code completion should include suggestions from scopes that are
-not visible. The required scope prefix will be inserted.
+not visible. The required scope prefix will be inserted. Default is `Yes`.
 
 ### ArgumentLists
 {:.v20}
@@ -394,6 +403,16 @@ The default is `FullPlaceholders`.
 This option governs the completion of template names as well, where
 the delimiters are `<>`.
 
+### HeaderInsertion
+{:.v21}
+
+Add `#include` directives when accepting code completions. Config
+equivalent of the CLI option `--header-insertion`. Valid values are:
+- `IWYU`: Include what you use. Insert the owning header for top-level
+    symbols, unless the header is already directly included or the
+    symbol is forward-declared. This is the default. 
+- `Never`: Never insert headers.
+
 ## InlayHints
 {:.v14}