Document user-level functions in project.el

* lisp/progmodes/project.el (project-find-file)
(project-or-external-find-file): Add doc strings.

etc/NEWS

@@ -964,13 +964,14 @@ As a result of this, the following commands are now obsolete:
 The framework's Lisp API is still experimental and can change in major,
 backward-incompatible ways.
 
+---
 ** New package Project
 
 The new package Project provides generic infrastructure for dealing
 with projects.  The main commands included in it are
 `project-find-file' and `project-find-regexp'.
 
-Like Xref, the project API is still experimental.
+The Lisp API of this package is still experimental.
 
 *** New variables
 

lisp/progmodes/project.el

@@ -313,6 +313,10 @@ pattern to search for."
 
 ;;;###autoload
 (defun project-find-file ()
+  "Visit a file in the current project's roots.
+
+This is like `find-file', but it limits the file-name completion
+candidates to the files within the current project roots."
   (interactive)
   (let* ((pr (project-current t))
          (dirs (project-roots pr)))
@@ -320,6 +324,10 @@ pattern to search for."
 
 ;;;###autoload
 (defun project-or-external-find-file ()
+  "Visit a file in the current project's roots or external roots.
+
+This is like `find-file', but it limits the file-name completion
+candidates to the files within the current project roots and external roots."
   (interactive)
   (let* ((pr (project-current t))
          (dirs (append