update: clarify docs

alldocs.go

diff --git a/alldocs.go b/alldocs.go
index 1461e7d..293e44b 100644
--- a/alldocs.go
+++ b/alldocs.go
@@ -67,21 +67,21 @@
 Update a local dependency
 
 Usage:
-        gvt update [-all] import
+        gvt update [ -all | importpath ]
 
-update will replaces the source with the latest available from the head of the master branch.
+update replaces the source with the latest available from the head of the fetched branch.
 
-Updating from one copy of a dependency to another comes with several restrictions.
-The first is you can only update to the head of the branch your dependency was vendored from, switching branches is not supported.
-The second restriction is if you have used -tag or -revision while vendoring a dependency, your dependency is "headless"
-(to borrow a term from git) and cannot be updated.
+Updating from one copy of a dependency to another is ONLY possible when the
+dependency was fetched by branch, without using -tag or -revision. It will be
+updated to the HEAD of that branch, switching branches is not supported.
 
-To update across branches, or from one tag/revision to another, you must first use delete to remove the dependency, then
-fetch [-tag | -revision | -branch ] [-precaire] to replace it.
+To update across branches, or from one tag/revision to another, you must first
+use delete to remove the dependency, then fetch [ -tag | -revision | -branch ]
+to replace it.
 
 Flags:
 	-all
-		will update all dependencies in the manifest, otherwise only the dependency supplied.
+		update all dependencies in the manifest.
 	-precaire
 		allow the use of insecure protocols.
 

update.go

diff --git a/update.go b/update.go
index a7d4d43..41b2fa4 100644
--- a/update.go
+++ b/update.go
@@ -21,30 +21,30 @@
 
 var cmdUpdate = &Command{
 	Name:      "update",
-	UsageLine: "update [-all] import",
+	UsageLine: "update [ -all | importpath ]",
 	Short:     "update a local dependency",
-	Long: `update will replaces the source with the latest available from the head of the master branch.
+	Long: `update replaces the source with the latest available from the head of the fetched branch.
 
-Updating from one copy of a dependency to another comes with several restrictions.
-The first is you can only update to the head of the branch your dependency was vendored from, switching branches is not supported.
-The second restriction is if you have used -tag or -revision while vendoring a dependency, your dependency is "headless"
-(to borrow a term from git) and cannot be updated.
+Updating from one copy of a dependency to another is ONLY possible when the
+dependency was fetched by branch, without using -tag or -revision. It will be
+updated to the HEAD of that branch, switching branches is not supported.
 
-To update across branches, or from one tag/revision to another, you must first use delete to remove the dependency, then
-fetch [-tag | -revision | -branch ] [-precaire] to replace it.
+To update across branches, or from one tag/revision to another, you must first
+use delete to remove the dependency, then fetch [ -tag | -revision | -branch ]
+to replace it.
 
 Flags:
 	-all
-		will update all dependencies in the manifest, otherwise only the dependency supplied.
+		update all dependencies in the manifest.
 	-precaire
 		allow the use of insecure protocols.
 
 `,
 	Run: func(args []string) error {
 		if len(args) != 1 && !updateAll {
-			return fmt.Errorf("update: import path or --all flag is missing")
+			return fmt.Errorf("update: import path or -all flag is missing")
 		} else if len(args) == 1 && updateAll {
-			return fmt.Errorf("update: you cannot specify path and --all flag at once")
+			return fmt.Errorf("update: you cannot specify path and -all flag at once")
 		}
 
 		m, err := vendor.ReadManifest(manifestFile())