[docs] Improve build instructions related to GC.

This attempts to fix #716.
Merge pull request #826 from waywardmonkeys/improve-gc-build-notes

[docs] Improve build instructions related to GC.
[make] install/release must do a 3-stage-bootstrap

There's no need for @bootstrap_target@ substitution here as doing
a 3-stage-bootstrap is required for these.
[configure] Default to doing a 3-stage-bootstrap.
[debian] Don't need to specify 3-stage-bootstrap.

Doing a full bootstrap is now the default behavior.