Add note on concurrency limit config to concurrency.md

[davidfrickert]

Description

This PR adds information on concurrency.md about the existing concurrency limit config that needs to be changed if one wants to enable concurrency.

Related issue and scope

• [ ] I opened an issue to propose and discuss this change (#????)

My changes affect the following components

• [x] Documentation

Types of changes

• [ ] Bug fix (generally a non-breaking change which closes an issue).
• [ ] Enhancement or new feature (adds new functionality).
• [ ] Breaking change (a bug fix or enhancement which changes existing behavior).

Checklist:

• [ ] I signed an Apache CLA.
• [ ] I reviewed the style guides and followed the recommendations (Travis CI will check :).
• [ ] I added tests to cover my changes.
• [ ] My changes require further changes to the documentation.
• [x] I updated the documentation where necessary.

[codecov-commenter]

Codecov Report

Merging #5103 (62380a2) into master (cd6fded) will decrease coverage by 27.94%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #5103       +/-   ##
==========================================
- Coverage   32.52%   4.58%   -27.95%     
==========================================
  Files         211     238       +27     
  Lines       10435   14178     +3743     
  Branches      447     579      +132     
==========================================
- Hits         3394     650     -2744     
- Misses       7041   13528     +6487     

Impacted Files	Coverage Δ
...a/org/apache/openwhisk/core/controller/Rules.scala	0.00% <0.00%> (ø)
...hisk/core/containerpool/docker/ProcessRunner.scala	0.00% <0.00%> (-46.67%)	:arrow_down:
...he/openwhisk/core/invoker/InstanceIdAssigner.scala	0.00% <0.00%> (ø)
...pache/openwhisk/core/entitlement/Entitlement.scala	0.00% <0.00%> (ø)
...enwhisk/core/entitlement/ActivationThrottler.scala	0.00% <0.00%> (ø)
...nwhisk/core/scheduler/queue/ContainerCounter.scala	0.00% <0.00%> (-84.22%)	:arrow_down:
...g/apache/openwhisk/core/database/UserCommand.scala	0.00% <0.00%> (ø)
...ain/scala/org/apache/openwhisk/spi/SpiLoader.scala	0.00% <0.00%> (-100.00%)	:arrow_down:
...n/scala/org/apache/openwhisk/utils/JsHelpers.scala	0.00% <0.00%> (ø)
...scala/org/apache/openwhisk/common/Prometheus.scala	0.00% <0.00%> (-23.08%)	:arrow_down:
... and 222 more

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[bdoyle0182]

LGTM

[davidfrickert]

@bdoyle0182 thanks for merging, haven't been checking on GitHub in a while and missed the approval