database/sql: document that drivers may not return right after cancel

Fixes #18168

Change-Id: Idbfe3d4daedd93c7caf6f1770ecd988e9af39949
Reviewed-on: https://go-review.googlesource.com/34144Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -8,7 +8,10 @@
 // The sql package must be used in conjunction with a database driver.
 // See https://golang.org/s/sqldrivers for a list of drivers.
 //
-// For more usage examples, see the wiki page at
+// Drivers that do not support context cancelation will not return until
+// after the query is completed.
+//
+// For usage examples, see the wiki page at
 // https://golang.org/s/sqlwiki.
 package sql