Update Bitraverse docs

[Zelenya]

• update bisequence doc (not sure if it got any better)
• add bitraverse doctest example

[LukaJCB]

Looks good to me, thanks! :)

[codecov-io]

Codecov Report

Merging #2291 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2291      +/-   ##
==========================================
+ Coverage   94.97%   95.01%   +0.03%     
==========================================
  Files         337      337              
  Lines        5832     5832              
  Branches      217      217              
==========================================
+ Hits         5539     5541       +2     
+ Misses        293      291       -2

Impacted Files	Coverage Δ
core/src/main/scala/cats/Bitraverse.scala	80% <ø> (ø)	⬆️
core/src/main/scala/cats/syntax/bitraverse.scala	100% <0%> (+50%)	⬆️

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[ceedubs]

Thanks @Zelenya!

I could see there being some merit in using different types instead of both inputs being Int and both outputs being Option[Int] to make it clear that this works on different types. But I think that this is a simple example and it's a nice addition.

[Zelenya]

@ceedubs yeah, I've initially did parseInt and parseBoolean, but those preparations took more space than example itself.