Clarify the behavior of wildcards in .dockerignore file

Unlike .gitignore, `*.md` in .dockerignore doesn't match
`subdir/foo.md`. While the logic is in github.com/moby/patternmatcher,
it is worth to note the difference in the reference document.

Signed-off-by: Kazuyoshi Kato <[email protected]>

Author: kzys

frontend/dockerfile/docs/reference.md

@@ -392,7 +392,9 @@ uses this mechanism:
 !README.md
 ```
 
-All markdown files *except* `README.md` are excluded from the context.
+All markdown files right under the context directory *except* `README.md`
+are excluded from the context. Note that markdown files under subdirectories
+are still included.
 
 The placement of `!` exception rules influences the behavior: the last
 line of the `.dockerignore` that matches a particular file determines