cmd/go/internal/{run,work,generate}: document file path location requirement

Mention that .go files must be from a single directory
for them to be treated as a single package.

Fixes #21529

Change-Id: I79cb08b9f43888814b1249a7b50bc7bc70bc1c72
Reviewed-on: https://go-review.googlesource.com/c/go/+/149797
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: agnivade

src/cmd/go/alldocs.go

@@ -110,8 +110,8 @@ specifies that the command "foo" represents the generator
 "go tool foo".
 
 Generate processes packages in the order given on the command line,
-one at a time. If the command line lists .go files, they are treated
-as a single package. Within a package, generate processes the
+one at a time. If the command line lists .go files from a single directory,
+they are treated as a single package. Within a package, generate processes the
 source files in a package in file name order, one at a time. Within
 a source file, generate runs generators in the order they appear
 in the file, one at a time.

src/cmd/go/internal/generate/generate.go

@@ -22,7 +22,7 @@ var CmdRun = &base.Command{
 	Short:     "compile and run Go program",
 	Long: `
 Run compiles and runs the named main Go package.
-Typically the package is specified as a list of .go source files,
+Typically the package is specified as a list of .go source files from a single directory,
 but it may also be an import path, file system path, or pattern
 matching a single known package, as in 'go run .' or 'go run my/cmd'.
 

src/cmd/go/internal/run/run.go

@@ -27,8 +27,8 @@ var CmdBuild = &base.Command{
 Build compiles the packages named by the import paths,
 along with their dependencies, but it does not install the results.
 
-If the arguments to build are a list of .go files, build treats
-them as a list of source files specifying a single package.
+If the arguments to build are a list of .go files from a single directory,
+build treats them as a list of source files specifying a single package.
 
 When compiling a single main package, build writes
 the resulting executable to an output file named after