Argument `interval` in `avg_syllables_per_word`

[LKirst]

I'm currently trying to add documentation to all textstat methods (see #178). Can somebody tell me what the purpose of the interval argument is in the avg_syllables_per_word? The only place it is used is in the flesh_reading_ease function, where it is set to 100 if the language is in ['es', 'it'].

[alxwrd]

It appears to be an adjustment for the Flesch-Vacca and Fernandez Huerta Readability Formula added in #97.

Flesch = 206.835 - (84.6 * syllables/words) - (1.015 * words/sentences)
Huerta = 206.84 - (0.60 * syllables/words) - (1.02 * words/sentences)

It seems to be a way to adjust the syllable count to bring higher syllable languages into line with English. @lmaczulajtys, sorry for the ping about a 2 year old PR, but do you happen to remember the source for implementing the interval?