Incorrect description of separator usage in Environment::with_prefix()

[Zoufar]

Description of pub fn with_prefix(s: &str) reads following:
"A prefix with a separator of _ is tested to be present on each key before its considered to be part of the source environment."

In fact, proper formulation could be:
"A prefix with a separator set by pub fn prefix_separator() OR (if no prefix_separator set) set by pub fn separator() OR (if none of previous) of "_" is tested to be present on each key before its considered to be part of the source environment."

Also, there may be added following piece of text to the description of pub fn separator(mut self, s: &str):
"In case of prefix separator not being explicitly set by pub fn prefix_separator(), this character sequence will be used also as prefix separator."