Clarified command line flags for dvc gc (#1644)

* Clarified command line flags for dvc gc

* Formatted with black, more explanation in GC_HELP

* Fix typo in GC_HELP

* Style corrections

dvc/command/gc.py

@@ -46,7 +46,11 @@ class CmdGC(CmdBase):
 
 
 def add_parser(subparsers, parent_parser):
-    GC_HELP = "Collect garbage."
+    GC_HELP = (
+        "Collect garbage from your DVC cache. \n"
+        "Deletes all files in the cache which are not in use by the "
+        "specified git references (defaults to just HEAD)."
+    )
     gc_parser = subparsers.add_parser(
         "gc", parents=[parent_parser], description=GC_HELP, help=GC_HELP
     )
@@ -55,14 +59,14 @@ def add_parser(subparsers, parent_parser):
         "--all-branches",
         action="store_true",
         default=False,
-        help="Collect garbage for all branches.",
+        help="Keep data files for the tips of all git branches.",
     )
     gc_parser.add_argument(
         "-T",
         "--all-tags",
         action="store_true",
         default=False,
-        help="Collect garbage for all tags.",
+        help="Keep data files for all git tags.",
     )
     gc_parser.add_argument(
         "-c",
@@ -79,7 +83,7 @@ def add_parser(subparsers, parent_parser):
         "--force",
         action="store_true",
         default=False,
-        help="Force garbage collection.",
+        help="Force garbage collection - automatically agree to all prompts.",
     )
     gc_parser.add_argument(
         "-j",
@@ -95,6 +99,8 @@ def add_parser(subparsers, parent_parser):
         type=str,
         nargs="*",
         default=None,
-        help="Collect garbage for all given projects.",
+        help="Keep data files required by these projects "
+        "in addition to the current one. "
+        "Useful if you share a single cache across repos.",
     )
     gc_parser.set_defaults(func=CmdGC)