Update docs about calling Glean.initialize first

[mdboom]

As @hawkinsw pointed out, this is no longer true and potentially confusing...

[codecov-io]

Codecov Report

Merging #473 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master    #473   +/-   ##
========================================
  Coverage      76.7%   76.7%           
  Complexity      330     330           
========================================
  Files            96      96           
  Lines          5558    5558           
  Branches        631     631           
========================================
  Hits           4263    4263           
  Misses          831     831           
  Partials        464     464

Impacted Files	Coverage Δ	Complexity Δ
...oid/src/main/java/mozilla/telemetry/glean/Glean.kt	85.48% <ø> (ø)	1 <0> (ø)	⬇️
glean-core/ios/Glean/Glean.swift	92.35% <ø> (ø)	0 <0> (ø)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 19ecf42...e4f0d7b. Read the comment docs.