Fix: Update docs for logcli

[nvtkaszpir]

What this PR does / why we need it:

Fix description for logcli, use proper flag names.
Extend docs with example using timestamp.

Which issue(s) this PR fixes:
Maybe not fixes but helps to acheive log download from loki described in #409

Special notes for your reviewer:
I tried to use command as described but there was an issue that:

• there were unrecognized flags (--start and --end were replaced with --from and --to)
• the timestamp message was pretty vague, apparetnly logcli appends timezone string, so the --from and --to are not exackly RFC3339Nano but more like RFC3339Nano without timezone.

Checklist

• [v] Documentation added

[CLAassistant]

All committers have signed the CLA.

[owen-d]

One small suggestion, but LGTM

[owen-d]

Thanks for the contribution!

[codecov-io]

Codecov Report

Merging #3212 (aa6a354) into master (e74eb54) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3212      +/-   ##
==========================================
+ Coverage   63.16%   63.21%   +0.04%     
==========================================
  Files         196      196              
  Lines       16668    16668              
==========================================
+ Hits        10529    10536       +7     
+ Misses       5187     5179       -8     
- Partials      952      953       +1     

Impacted Files	Coverage Δ
pkg/canary/comparator/comparator.go	76.36% <0.00%> (-1.82%)	⬇️
pkg/promtail/positions/positions.go	58.51% <0.00%> (+11.70%)	⬆️