Clarify the documentation for url::{AddStandardScheme,IsStandard}.

url/gurl.h

Index: url/gurl.h
diff --git a/url/gurl.h b/url/gurl.h
index b9136b81f6a8b962a79d3e0435addc614a1e6bb0..c0ad418add981f2e56644f35dcb14dde757aaf77 100644
--- a/url/gurl.h
+++ b/url/gurl.h
@@ -194,10 +194,11 @@ class URL_EXPORT GURL {
   // returned.
   GURL GetAsReferrer() const;
 
-  // Returns true if the scheme for the current URL is a known "standard"
-  // scheme. Standard schemes have an authority and a path section. This
-  // includes file: and filesystem:, which some callers may want to filter out
-  // explicitly by calling SchemeIsFile[System].
+  // Returns true if the scheme for the current URL is a known "standard-format"
+  // scheme. A standard-format scheme adheres to what RFC 3986 calls "generic
+  // URI syntax" (https://tools.ietf.org/html/rfc3986#section-3). This includes
+  // file: and filesystem:, which some callers may want to filter out explicitly
+  // by calling SchemeIsFile[System].
   bool IsStandard() const;
 
   // Returns true if the given parameter (should be lower-case ASCII to match