docs: service create: document non-matching/exclude constraints

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit b059c93c9211a0bcc8aea0ffdea1f591aa0d853a)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 27f061266579ea6402feb047b24ab66463ef69d7
Component: cli

Author: thaJeztah

components/cli/docs/reference/commandline/service_create.md

@@ -667,7 +667,8 @@ $ docker service create \
 ### Specify service constraints (--constraint)
 
 You can limit the set of nodes where a task can be scheduled by defining
-constraint expressions. Multiple constraints find nodes that satisfy every
+constraint expressions. Constraint expressions can either use a _match_ (`==`)
+or _exclude_ (`!=`) rule. Multiple constraints find nodes that satisfy every
 expression (AND match). Constraints can match node or Docker Engine labels as
 follows: