embed: document additional file name restrictions

For #44486

Change-Id: I66af9f7a9f95489a41fd6710e50bdd7878f78b85
Reviewed-on: https://go-review.googlesource.com/c/go/+/413494
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/embed/embed.go

@@ -91,6 +91,7 @@
 // It can only be used with variables at package scope, not with local variables.
 //
 // Patterns must not match files outside the package's module, such as ‘.git/*’ or symbolic links.
+// Patterns must not match files whose names include the special punctuation characters  " * < > ? ` ' | / \ and :.
 // Matches for empty directories are ignored. After that, each pattern in a //go:embed line
 // must match at least one file or non-empty directory.
 //