Update docs for running locust in Step Load Mode

[delulu]

Change in this PR:

1. Update docs for running locust in Step Load Mode

2. Fix step load bug in local run

   When running locust in standalone way, self.greenlet will point to self.locusts. So we can't use this gevent Group to spawn the stepload_greenlet, otherwise it will be wrongly taken as the running locust.

[codecov]

Codecov Report

Merging #1235 into master will decrease coverage by 0.1%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1235      +/-   ##
==========================================
- Coverage   78.77%   78.67%   -0.11%     
==========================================
  Files          20       20              
  Lines        1955     1955              
  Branches      308      308              
==========================================
- Hits         1540     1538       -2     
- Misses        335      336       +1     
- Partials       80       81       +1

Impacted Files	Coverage Δ
locust/runners.py	78.33% <50%> (ø)	⬆️
locust/util/deprecation.py	85% <0%> (ø)	⬆️
locust/contrib/fasthttp.py	91.17% <0%> (ø)	⬆️
locust/web.py	89.05% <0%> (ø)	⬆️
locust/main.py	34.42% <0%> (ø)	⬆️
locust/stats.py	84.35% <0%> (-0.46%)	⬇️
locust/core.py	87.71% <0%> (ø)	⬆️

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[cyberw]

Thanks!