database/sql: add description to String method of IsolationLevel struct.
database/sql: add description to String method of IsolationLevel struct.

Add simple description to String method of IsolationLevel struct.

Change-Id: I8bdf829c81d4895b8542a3f21437bed61c6e925d
GitHub-Last-Rev: 7e16d6a93a9560cf3a09413dfc47d0cb04bb2fde
GitHub-Pull-Request: golang/go#28560
Reviewed-on: https://go-review.googlesource.com/c/147040
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,nawawi/go,golang/go,akutz/go,golang/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,golang/go,akutz/go,josharian/go,josharian/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,josharian/go,CAFxX/go,josharian/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,josharian/go,cloudflare/go,akutz/go,cloudflare/go,akutz/go,josharian/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,akutz/go,josharian/go,nawawi/go