Missing details for bit array options

[brettcannon]

This is a more generic response to #57 .

https://tour.gleam.run/data-types/bit-arrays/ lists various options, but they don't specify things like which ones require an argument (like size), ones that require more than just themselves (e.g. unit), how to change int to not be "a default size of 8 bits", etc.

The list is also missing:

• utf8_codepoint
• utf16_codepoint
• utf32_codepoint
• signed
• unsigned based on the error message.

[lpil]

Thank you