Update copypasta line (#529) (#530)

* Update copypasta line

I'm note sure what goes in `???` but just to get the PR rolling.

* Fill in the text for manifest mode

Based on what I see at https://docs.julialang.org/en/latest/stdlib/Pkg/#Adding-registered-packages-1 this text seems correct.

* Editorial fix

stdlib/Pkg/src/REPLMode.jl

@@ -440,8 +440,8 @@ const helps = Dict(
     the project file is summarized, showing what version each package is on and
     how it has changed since the last git commit (if in a git repo), as well as
     any changes to manifest packages not already listed. In `--project` mode, the
-    status of the project file is summarized. In `--project` mode, the status of
-    the project file is summarized.
+    status of the project file is summarized. In `--manifest` mode the output also
+    includes the dependencies of explicitly added packages.
     """, CMD_GENERATE => md"""
 
         generate pkgname