Minor jointplot user-friendliness

[mwaskom]

• Warn/ignore when ax is passed as kwarg (closes jointplot doesn't warn for ax= argument in seaborn 11.2 #2666)
• Set the joint axes to be active (closes suggestion: set current ax to g.ax_joint at end of jointplot #2511)

[codecov]

Codecov Report

Merging #2863 (e58c8f4) into master (a48dc8f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2863      +/-   ##
==========================================
+ Coverage   98.11%   98.13%   +0.01%     
==========================================
  Files          69       69              
  Lines       22247    22252       +5     
==========================================
+ Hits        21828    21836       +8     
+ Misses        419      416       -3     

Impacted Files	Coverage Δ
seaborn/axisgrid.py	97.17% <100.00%> (+0.29%)	⬆️
tests/test_axisgrid.py	99.36% <100.00%> (+<0.01%)	⬆️