Clarify `cp` documentation behaviour with trailing "/."

Issue #30082 demonstrated that their is possible confusion with the "/."
where the tailing "." can appear to be merely punctuation within the
document rather than a highly pertinent part of `SRC_PATH`.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

docs/reference/commandline/cp.md

@@ -75,9 +75,9 @@ argument of `DEST_PATH`, the behavior is as follows:
     - `DEST_PATH` exists and is a file
         - Error condition: cannot copy a directory to a file
     - `DEST_PATH` exists and is a directory
-        - `SRC_PATH` does not end with `/.`
+        - `SRC_PATH` does not end with `/.` (that is: _slash_ followed by _dot_)
             - the source directory is copied into this directory
-        - `SRC_PATH` does end with `/.`
+        - `SRC_PATH` does end with `/.` (that is: _slash_ followed by _dot_)
             - the *content* of the source directory is copied into this
               directory
 

man/src/container/cp.md

@@ -42,9 +42,9 @@ argument of `DEST_PATH`, the behavior is as follows:
     - `DEST_PATH` exists and is a file
         - Error condition: cannot copy a directory to a file
     - `DEST_PATH` exists and is a directory
-        - `SRC_PATH` does not end with `/.`
+        - `SRC_PATH` does not end with `/.` (that is: _slash_ followed by _dot_)
             - the source directory is copied into this directory
-        - `SRC_PATH` does end with `/.`
+        - `SRC_PATH` does end with `/.` (that is: _slash_ followed by _dot_)
             - the *content* of the source directory is copied into this
               directory