chapter9: optional: footnote that curl can make non-HTTP requests

curl can make HTTP requests. It can also make requests of other types.
This commit proposes adding a footnote to note that curl can make
other types of requests.

Options for manual page:
 - https://man7.org/linux/man-pages/man1/curl.1.html
 - https://linux.die.net/man/1/curl

Author: zipperer

chapter_09_docker.asciidoc

@@ -956,7 +956,7 @@ Starting development server at http://127.0.0.1:8888/
 
 A quick run of the FT or check in our browser will show us that nope, that doesn't work either.
 Let's try an even lower-level smoke test, the traditional Unix utility `curl`.
-It's a command-line tool for making HTTP requests.  Try it on your own computer first:
+It's a command-line tool for making HTTP requests.footnote:[`curl` can make many other types of requests. For details see https://man7.org/linux/man-pages/man1/curl.1.html[curl manual]]  Try it on your own computer first:
 
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