[docs] Add set placement endpoint docs

[benraskin92]

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 #2183 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2183     +/-   ##
========================================
+ Coverage    72.2%   72.3%   +0.1%     
========================================
  Files        1017    1016      -1     
  Lines       88265   88056    -209     
========================================
- Hits        63739   63693     -46     
+ Misses      20239   20086    -153     
+ Partials     4287    4277     -10

Flag	Coverage Δ
#aggregator	82% <0%> (ø)	⬆️
#cluster	85.2% <0%> (-0.2%)	⬇️
#collector	82.8% <0%> (ø)	⬆️
#dbnode	79.2% <0%> (+0.2%)	⬆️
#m3em	74.4% <0%> (ø)	⬆️
#m3ninx	74.2% <0%> (+46.9%)	⬆️
#m3nsch	51.1% <0%> (ø)	⬆️
#metrics	17.6% <0%> (ø)	⬆️
#msg	74.9% <0%> (+0.1%)	⬆️
#query	68.3% <0%> (+0.1%)	⬆️
#x	83% <0%> (+0.2%)	⬆️

---

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[codecov]

Codecov Report

Merging #2183 into master will decrease coverage by 0.4%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2183     +/-   ##
========================================
- Coverage    72.3%   71.8%   -0.5%     
========================================
  Files        1016    1016             
  Lines       88056   88056             
========================================
- Hits        63701   63309    -392     
- Misses      20082   20504    +422     
+ Partials     4273    4243     -30     

Flag	Coverage Δ
#aggregator	82.0% <ø> (-0.1%)	⬇️
#cluster	85.2% <ø> (-0.2%)	⬇️
#collector	82.8% <ø> (ø)	⬆️
#dbnode	79.2% <ø> (+<0.1%)	⬆️
#m3em	74.4% <ø> (ø)	⬆️
#m3ninx	63.6% <ø> (-10.7%)	⬇️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	52.2% <ø> (+34.5%)	⬆️
#msg	74.8% <ø> (-0.3%)	⬇️
#query	68.3% <ø> (ø)	⬆️
#x	83.0% <ø> (-0.1%)	⬇️

---

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[robskillington]

LGTM other than minor bit about removing mention of “seed node” and just mentioning “nodes”