Update the base::UnguessableToken documentation.

Use terminology consistent with the literature.

Bug: None
Change-Id: I3bcb1a747f23ad9418695926107b57eaa097ad41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1808303Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697325}

base/unguessable_token.h

@@ -20,26 +20,30 @@ namespace base {
 struct UnguessableTokenHash;
 
 // UnguessableToken is, like Token, a randomly chosen 128-bit value. Unlike
-// Token however, a new UnguessableToken must always be generated at runtime
-// from a cryptographically strong random source (or copied or serialized and
+// Token, a new UnguessableToken is always generated at runtime from a
+// cryptographically strong random source (or copied or serialized and
 // deserialized from another such UnguessableToken). It can be used as part of a
 // larger aggregate type, or as an ID in and of itself.
 //
-// UnguessableToken can be used to implement "Capability-Based Security".
-// In other words, UnguessableToken can be used when the resource associated
-// with the ID needs to be protected against manipulation by other untrusted
-// agents in the system, and there is no other convenient way to verify the
-// authority of the agent to do so (because the resource is part of a table
-// shared across processes, for instance). In such a scheme, knowledge of the
-// token value in and of itself is sufficient proof of authority to carry out
-// an operation against the associated resource.
+// An UnguessableToken is a strong *bearer token*. Bearer tokens are like HTTP
+// cookies: if a caller has the token, the callee thereby considers the caller
+// authorized to request the operation the callee performs.
+//
+// UnguessableToken can be used when the resource associated with the ID needs
+// to be protected against manipulation by other untrusted agents in the system,
+// and there is no other convenient way to verify the authority of the agent to
+// do so (because the resource is part of a table shared across processes, for
+// instance). In such a scheme, knowledge of the token value in and of itself is
+// sufficient proof of authority to carry out an operation on the associated
+// resource.
 //
 // Use Create() for creating new UnguessableTokens.
 //
 // NOTE: It is illegal to send empty UnguessableTokens across processes, and
-// sending/receiving empty tokens should be treated as a security issue.
-// If there is a valid scenario for sending "no token" across processes,
-// base::Optional should be used instead of an empty token.
+// sending/receiving empty tokens should be treated as a security issue. If
+// there is a valid scenario for sending "no token" across processes, use
+// base::Optional instead of an empty token.
+
 class BASE_EXPORT UnguessableToken {
  public:
   // Create a unique UnguessableToken.