term: avoid "under X control" and other help output updates (#3322)

* get/import: update cmd and url arg descs with correct behavior

* get/import: update path arg desc as well (See previous commit)

* get/imp: udpate help output with simpler terms
per https://github.com/iterative/dvc/pull/3277#pullrequestreview-352650016

* api: url->URL in a docstring

* metrics show: update help output per
per https://github.com/iterative/dvc.org/pull/933#pullrequestreview-351862579

* diff: update `diff` and `metrics diff` help desc to match docs
per https://github.com/iterative/dvc.org/pull/933#pullrequestreview-356890960

* diff: update autocomplete scripts
per https://github.com/iterative/dvc/pull/3308#issuecomment-585220300

* diff: update help output again
per https://github.com/iterative/dvc.org/pull/933#pullrequestreview-356890960

* diff/metrics diff: update help output yet again
per https://github.com/iterative/dvc.org/pull/933#discussion_r378654316

* term" under X control -> tracked by X

dvc/api.py

@@ -7,7 +7,7 @@ from dvc.external_repo import external_repo
 
 
 class UrlNotDvcRepoError(DvcException):
-    """Thrown if given url is not a DVC repository.
+    """Thrown if given URL is not a DVC repository.
 
     Args:
         url (str): URL to the repository

dvc/command/data_sync.py

@@ -148,7 +148,8 @@ def add_parser(subparsers, _parent_parser):
     pull_parser.set_defaults(func=CmdDataPull)
 
     # Push
-    PUSH_HELP = "Push data files to a DVC remote storage."
+    PUSH_HELP = "Uploads files or directories tracked by DVC"
+    " to remote storage."
 
     push_parser = subparsers.add_parser(
         "push",
@@ -191,7 +192,8 @@ def add_parser(subparsers, _parent_parser):
     push_parser.set_defaults(func=CmdDataPush)
 
     # Fetch
-    FETCH_HELP = "Fetch data files from a DVC remote storage."
+    FETCH_HELP = "Get files or directories tracked by DVC from remote storage"
+    " into the cache."
 
     fetch_parser = subparsers.add_parser(
         "fetch",

dvc/command/diff.py

@@ -120,8 +120,9 @@ class CmdDiff(CmdBase):
 
 def add_parser(subparsers, parent_parser):
     DIFF_DESCRIPTION = (
-        "Compare two versions of the DVC repository.\n"
-        "Shows the list of paths added, modified, or deleted"
+        "Show changes between commits in the DVC repository,"
+        " or between a commit and the workspace."
+        "Prints the list of paths added, modified, or deleted."
     )
     diff_parser = subparsers.add_parser(
         "diff",

dvc/command/metrics.py

@@ -273,8 +273,8 @@ def add_parser(subparsers, parent_parser):
     metrics_remove_parser.add_argument("path", help="Path to a metric file.")
     metrics_remove_parser.set_defaults(func=CmdMetricsRemove)
 
-    METRICS_DIFF_HELP = "Show a table of changes between metrics among "
-    "versions of the DVC repository."
+    METRICS_DIFF_HELP = "Show changes in metrics between commits"
+    " in the DVC repository, or between a commit and the workspace."
     metrics_diff_parser = metrics_subparsers.add_parser(
         "diff",
         parents=[parent_parser],

scripts/completion/dvc.zsh

@@ -12,14 +12,14 @@
 
 _dvc_commands() {
   local _commands=(
-    "add:Take data files or directories under DVC control."
+    "add:Track data files or directories with DVC."
     "cache:Manage cache settings."
     "checkout:Checkout data files from cache."
     "commit:Save changed data to cache and update DVC-files."
     "config:Get or set config settings."
     "destroy:Remove DVC-files, local DVC config and data cache."
-    "diff:Show a diff of a DVC controlled data file or a directory."
-    "fetch:Fetch data files from a DVC remote storage."
+    "diff:Show changes between commits in the DVC repository, or between a commit and the workspace."
+    "fetch:Get files or directories tracked by DVC from remote storage into the cache."
     "get-url:Download or copy files from URL."
     "get:Download data from DVC repository."
     "gc:Garbage collect unused objects from cache or remote storage."
@@ -100,8 +100,8 @@ _dvc_destroy=(
 _dvc_diff=(
   "--show-json[Format the output into a JSON]"
   "--checksums[Display checksums for each entry]"
-  "1:Git reference to the older version:"
-  "2:Git reference to the newer version:"
+  "1:Old Git commit to compare (defaults to HEAD):"
+  "2:New Git commit to compare (defaults to the current workspace):"
 )
 
 _dvc_fetch=(