docs(picker): document unified readable labels

Author: mhiro2

README.md

@@ -272,6 +272,8 @@ default backend is `builtin`. To use an external picker, install the plugin and
 `picker.backend` to one of: `telescope`, `fzf-lua`, `snacks`.
 When using these external backends, the picker preview window shows the selected file
 content around the target location.
+Candidate labels are shown in a readable unified format:
+`<text> - <path>:<line>:<col>` (or `<path>:<line>:<col>` when text is empty).
 
 ```lua
 {

doc/peekstack.txt

@@ -155,6 +155,10 @@ Configure the backend with `picker.backend`:
 
 For external backends (`telescope`, `fzf-lua`, `snacks`), picker preview windows
 show file content around the selected location.
+Picker labels use a unified readable format:
+    <text> - <path>:<line>:<col>
+or, when text is empty:
+    <path>:<line>:<col>
 
 If the chosen plugin is not installed, a warning is shown and the picker will not open.