Rollup merge of #128902 - evanj:evan.jones/env-var-doc, r=workingjubilee

doc: std::env::var: Returns None for names with '=' or NUL byte

The documentation incorrectly stated that std::env::var could return an error for variable names containing '=' or the NUL byte. Copy the correct documentation from var_os.

var_os was fixed in Commit 8a7a665, Pull Request #109894, which closed Issue #109893.

This documentation was incorrectly added in commit f2c0f292, which replaced a panic in var_os by returning None, but documented the change as "May error if ...".

Reference the specific error values and link to them.

Author: tgross35