doc: improve the garbage collection discussion
doc: improve the garbage collection discussion

Bring it up to date with recent terminology, mention new approaches
such as in Rust, and link to the new blog post.

Change-Id: I1d0b121e6f8347c3cf2c8ca0d8adc8285ce59ef1
Reviewed-on: https://go-review.googlesource.com/124475
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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