Minor updates to the external Node document

[jianjuns]

@wenyingd : when reading the doc, I feel these bold font sentences look strange, and also feel not necessary to emphasize them. Let me know what you think.

[wenyingd]

I thought thoses sentences were some key points needed to remind users, that is why I emphasized them. I am OK if you think it is not necessary and remove the mark.

[wenyingd]

LGTM

[jianjuns]

I actually got feedback from a reader that he felt he missed some important points after reading the sentences. And these two are actually about internal implementation, which users may not need to care.

[codecov]

Codecov Report

Merging #4102 (a4fe326) into feature/externalnode (274bb85) will increase coverage by 2.99%.
The diff coverage is n/a.

@@                   Coverage Diff                    @@
##           feature/externalnode    #4102      +/-   ##
========================================================
+ Coverage                 62.66%   65.65%   +2.99%     
========================================================
  Files                       299      302       +3     
  Lines                     45991    46085      +94     
========================================================
+ Hits                      28819    30256    +1437     
+ Misses                    14819    13445    -1374     
- Partials                   2353     2384      +31     

Flag	Coverage Δ		*Carryforward flag
integration-tests	34.59% <ø> (?)
kind-e2e-tests	49.25% <ø> (ø)		Carriedforward from 274bb85
unit-tests	44.09% <ø> (ø)		Carriedforward from 274bb85

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
pkg/agent/cniserver/ipam/testing/utils.go	92.85% <0.00%> (ø)
...icluster/controllers/multicluster/common/helper.go	58.00% <0.00%> (ø)
pkg/agent/cniserver/testing/utils.go	100.00% <0.00%> (ø)
pkg/agent/util/net.go	45.69% <0.00%> (+0.74%)	⬆️
pkg/agent/openflow/network_policy.go	78.97% <0.00%> (+1.53%)	⬆️
pkg/agent/util/iptables/iptables.go	45.07% <0.00%> (+1.55%)	⬆️
pkg/agent/openflow/framework.go	89.92% <0.00%> (+2.15%)	⬆️
pkg/agent/cniserver/pod_configuration.go	57.66% <0.00%> (+3.06%)	⬆️
pkg/agent/openflow/client.go	69.10% <0.00%> (+3.40%)	⬆️
pkg/agent/types/networkpolicy.go	86.48% <0.00%> (+5.40%)	⬆️
... and 27 more

[wenyingd]

I actually got feedback from a reader that he felt he missed some important points after reading the sentences. And these two are actually about internal implementation, which users may not need to care.

Got it. I think the sentence about "L3 routing is not provided" is an internal implementation, but for the automatic convention from ExternalNode to ExternalEntity is actually what needs to be known by users, so that the users do not need to manually create ExternalEntity.

[jianjuns]

Sure. But even that one seems to me need not to highlight in the whole doc.