embed: document exclusions more explicitly

Fixes #45197
Fixes #45744

Change-Id: Ic221da1b652f693c7aa37287932c0a8726f46765
Reviewed-on: https://go-review.googlesource.com/c/go/+/633815
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: seankhliao

src/embed/embed.go

@@ -90,7 +90,8 @@
 // depending on whether the package wants to make the data available to other packages.
 // 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 outside the package's module, such as ‘.git/*’, symbolic links,
+// 'vendor/', or any directories containing go.mod (these are separate modules).
 // 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.