store: Clarify documentation of 'valid-path?'.

* guix/store.scm (valid-path?): Improve docstring.
* doc/guix.texi (The Store): Update accordingly.

doc/guix.texi

@@ -2913,7 +2913,14 @@ Procedures that make RPCs all take a server object as their first
 argument.
 
 @deffn {Scheme Procedure} valid-path? @var{server} @var{path}
-Return @code{#t} when @var{path} is a valid store path.
+@cindex invalid store items
+Return @code{#t} when @var{path} designates a valid store item and
+@code{#f} otherwise (an invalid item may exist on disk but still be
+invalid, for instance because it is the result of an aborted or failed
+build.)
+
+A @code{&nix-protocol-error} condition is raised if @var{path} is not
+prefixed by the store directory (@file{/gnu/store}).
 @end deffn
 
 @deffn {Scheme Procedure} add-text-to-store @var{server} @var{name} @var{text} [@var{references}]

guix/store.scm

@@ -582,7 +582,12 @@ encoding conversion errors."
     (operation (name args ...) docstring return ...)))
 
 (define-operation (valid-path? (string path))
-  "Return #t when PATH is a valid store path."
+  "Return #t when PATH designates a valid store item and #f otherwise (an
+invalid item may exist on disk but still be invalid, for instance because it
+is the result of an aborted or failed build.)
+
+A '&nix-protocol-error' condition is raised if PATH is not prefixed by the
+store directory (/gnu/store)."
   boolean)
 
 (define-operation (query-path-hash (store-path path))