DOC: Documentation cleanup for stats tutorials.

[pvanmulbregt]

• Be more explicit about the support of a distribution. Have it agree with the implementation.
• Added definitions of functions used (I -> incomplete beta, or incomplete gamma or bessel,...; airy's Hh_v)
• Removed indicator functions (inconsistently used, confusible with the I's above)
• Replace G(\alpha;...) with G(q; ...) and make follow-on changes:
  gengamma, levy_l, nakagami, dgamma, chi-squared, loggamma, gamma.
• Fixed ordering of arguments (student t)
• Reformatted many long (>1500chars) lines for maintainability.
• Fixed m_n median for double weibull.

[rgommers]

Be more explicit about the support of a distribution. Have it agree with
the implementation.

Good idea, thanks for reviewing all that.

The doc build CI failed due to some unresolved references.

[pvanmulbregt]

The unresolved symbols turned out to be a consequence of a failure in an earlier step of the build, due to some long-standing extraneous alignment tab characters.

[rgommers]

I reviewed a few bits in detail and sanity checked the rest. All looked great. The reformatting of the equations makes things more readable as well.

Merging. Thanks @pvanmulbregt!