docs: Update documentation for --install-branch option

aider/website/docs/config/aider_conf.md

@@ -358,8 +358,8 @@ cog.outl("```")
 ## Show release notes on first run of new version (default: None, ask user)
 #show-release-notes: xxx
 
-## Install the latest version from the main branch
-#install-main-branch: false
+## Install the latest version from a git branch. If GIT_URL is provided, installs from that URL (e.g., git+https://github.com/user/aider.git@my-branch). If no URL is provided, installs from the Aider-AI main branch.
+#install-branch: false
 
 ## Upgrade aider to the latest version from PyPI
 #upgrade: false

aider/website/docs/config/options.md

@@ -65,7 +65,7 @@ usage: aider [-h] [--model] [--openai-api-key] [--anthropic-api-key]
              [--analytics | --no-analytics] [--analytics-log]
              [--analytics-disable] [--just-check-update]
              [--check-update | --no-check-update]
-             [--show-release-notes | --no-show-release-notes]
+             [--show-release-notes | --no-show-release-notes] [--install-branch]
              [--install-main-branch] [--upgrade] [--version]
              [--message] [--message-file]
              [--gui | --no-gui | --browser | --no-browser]
@@ -559,10 +559,10 @@ Aliases:
   - `--show-release-notes`
   - `--no-show-release-notes`
 
-### `--install-main-branch`
-Install the latest version from the main branch  
+### `--install-branch [GIT_URL]`
+Install the latest version from a git branch. If GIT_URL is provided, installs from that URL (e.g., git+https://github.com/user/aider.git@my-branch). If no URL is provided, installs from the Aider-AI main branch.  
 Default: False  
-Environment variable: `AIDER_INSTALL_MAIN_BRANCH`  
+Environment variable: `AIDER_INSTALL_BRANCH`  
 
 ### `--upgrade`
 Upgrade aider to the latest version from PyPI