Fix the regex for file.uncomment checking

Anchor to beginning of line, allowing for leading spaces
Check for comment character when checking for commented regex
Update docs to reflect new uncomment behavior
Allow for tabs in leading whitespace in file.uncomment
Change file.uncomment to keep leading spaces

Previously deleted a single space after the comment character
Merge pull request #4015 from basepi/fileuncomment4003

Regex fixes for file.uncomment state, Fixes #4003