Document creating a nested/child span

[cartermp]

Tracking issue for the "Nested Spans" row here: https://docs.google.com/spreadsheets/d/1EO0HkjraZSN_7LlFpwFPoazR9xbdCCgQi-IFc0YA5rU/edit#gid=0

Emoji guide:

✔️ - this is basically good
➖ - this could be improved
❌ - this needs to exist or be rewritten

Currently documented with an example:

• ✔️ Java
• ✔️ C++
• ➖ Go (probably needs explicit section for the sample)
• ➖ JavaScript (probably needs explicit section for the code sample)
• ➖ Erlang/Elixir (exists but no explicit section)
• ➖ Python (it's in the quickstart but that's it, no section, no explaining the relationship)

Has no section at all:

• ❌ Ruby
• ❌ .NET
• ❌ Rust
• ❌ Swift
• ❌ PHP

[svrnm]

@cartermp, Can this be superseded by #3229?

[cartermp]

Yep, let's close in favor. Whew, this one was pretty old