file.jl: add a note differentiating cp function from cp command (J…

…uliaLang#41761)

The `cp` function behaves more deterministically than the command and
can have serious consequences if `force=true` when `dst` is a directory.
Add a note to the docstring mentioning this.

base/file.jl

@@ -359,6 +359,13 @@ If `follow_symlinks=false`, and `src` is a symbolic link, `dst` will be created
 symbolic link. If `follow_symlinks=true` and `src` is a symbolic link, `dst` will be a copy
 of the file or directory `src` refers to.
 Return `dst`.
+
+!!! note
+    The `cp` function is different from the `cp` command. The `cp` function always operates on
+    the assumption that `dst` is a file, while the command does different things depending
+    on whether `dst` is a directory or a file.
+    Using `force=true` when `dst` is a directory will result in loss of all the contents present
+    in the `dst` directory, and `dst` will become a file that has the contents of `src` instead.
 """
 function cp(src::AbstractString, dst::AbstractString; force::Bool=false,
                                                       follow_symlinks::Bool=false)