access: `*` for any one key in a capability's path

A capability's path covered by prefix only, so what is the same at every key of a map
(each node's log filter, say, at `_continuum/local/<id>/log` from the next patch on)
took a capability per key. Now a `*` segment stands for any one key, as it already does
in signed paths and sealed places (`envelope`, `sealed`, which now share one `agree`):

* covering (`Capability::covers`): the pattern no longer than the path, and agreeing
  with it segment by segment;
* listings (`reaches`): pattern and path agreeing on the segments both have, so every
  key on the way shows, nothing past where the pattern differs;
* narrowing (`Capability::holds`, for a token's limit narrowed again and a refresh's):
  at or below, a `*` only under a `*`: a limit naming one key never widens to all.

A `*` first is outside protocol data, so it never covers `_continuum`, as `[]` doesn't.
A key that is `*` itself can't be named alone. Every node judges changes by
capabilities, so all must read them alike: they run the same version.

1 files changed, 5 insertions, 3 deletions

diff --git a/docs/access.md b/docs/access.md
index 7392bb9..c639d0d 100644
--- a/docs/access.md
+++ b/docs/access.md
@@ -20,9 +20,11 @@ password hashes: every member reads the document).
 * **Names** are what `--htpasswd` and the proxy give: 1 to 64 of `A-Z a-z 0-9 . _ @ -`.
 * **Capabilities** (`caps`): each a path, as segments as the store's API takes them,
   and the rights it gives there and below: `read`, `write`. A capability covers its
-  path segment by segment (`servers/eu` covers `servers/eu/0`, not `servers/europe`).
-  One outside protocol data never covers protocol data: `[]` is all of the swarm's
-  data, `["_continuum"]` protocol data, managing users included (`_continuum/users`).
+  path segment by segment (`servers/eu` covers `servers/eu/0`, not `servers/europe`),
+  `*` for any one key (`servers/*/name` covers `servers/eu/name` and `servers/us/name`;
+  a key that is `*` itself can't be named alone). One outside protocol data never
+  covers protocol data, nor one with `*` first: `[]` is all of the swarm's data,
+  `["_continuum"]` protocol data, managing users included (`_continuum/users`).
   They are kept by a random ID, so two capabilities granted at once both hold, and
   revoking one removes it alone. Each names who granted it (`by`: a user as
   `user:<name>`, or the node that recorded it) and when (`since`).