[Docs] -- Adding clarification around etcd and M3 to etcd operational guide

[gibbscullen]

What this PR does / why we need it:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

Does this PR require updating code package or user-facing documentation?:

[codecov]

Codecov Report

Merging #3361 (796ac5e) into master (5d23e1b) will decrease coverage by 0.0%.
The diff coverage is n/a.

❗ Current head 796ac5e differs from pull request most recent head 334f11a. Consider uploading reports for the commit 334f11a to get more accurate results

@@            Coverage Diff            @@
##           master    #3361     +/-   ##
=========================================
- Coverage    72.3%    72.3%   -0.1%     
=========================================
  Files        1099     1099             
  Lines      102229   102224      -5     
=========================================
- Hits        74007    73969     -38     
- Misses      23116    23136     +20     
- Partials     5106     5119     +13     

Flag	Coverage Δ
aggregator	76.8% <ø> (-0.1%)	⬇️
cluster	84.9% <ø> (-0.1%)	⬇️
collector	84.3% <ø> (ø)
dbnode	78.9% <ø> (-0.1%)	⬇️
m3em	74.4% <ø> (ø)
m3ninx	73.5% <ø> (ø)
metrics	19.8% <ø> (ø)
msg	74.6% <ø> (ø)
query	66.9% <ø> (ø)
x	80.3% <ø> (-0.1%)	⬇️

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[ChrisChinchilla]

Looks good @gibbscullen I will overhaul this section at a later date, but this is a nice improvement for now.