API Docs: std::str

[steveklabnik]

Part of #29329

http://doc.rust-lang.org/std/str/

Here's what needs to be done to close out this issue:

• the module documentation is fine, but should point out why std::str and the primitive type are different.
• Bytes should use the same boilerplate as Lines Modify str Structs descriptions #40935 (review)
• CharIndices has the same issue. Modify str Structs descriptions #40935 (review)
• Chars has the same issue. Modify str Structs descriptions #40935 (review)
• EncodeUtf16 has the same issue. Modify str Structs descriptions #40935 (review)
• ParseBoolError should link to the method it comes from.
• SplitWhitespace should use the same boilerplate as Lines.
• Utf8Error should link to those functions.
• FromStr should have an example of implementing it.
• from_utf8 should link the types it mentions.
• from_utf8_unchecked should link that &str.

[steveklabnik]

As shown in #30057, there's a lot of junk there that could be in Pattern instead.

[steveklabnik]

I am happy to mentor anyone who wants to tackle this issue.

[tigleym]

Hello! I'd like to give this issue a try.

[steveklabnik]

@tigleym that'd be great! this issue is very general, so any improvements would be welcome. Let me know if you need any help!