Updated Names of Model-Adding Methods

[liangfok]

This builds upon #3043. It renames all of the methods that add models to the RigidBodySystem and its RigidBodyTree. It also adds a bunch of Doxygen comments.

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This change is

[liangfok]

@drake-jenkins-bot linux-gcc-experimental-ros please
@drake-jenkins-bot linux-clang-experimental-ros please
@drake-jenkins-bot linux-gcc-experimental-matlab-ros please

[liangfok]

@drake-jenkins-bot linux-gcc-experimental-ros please
@drake-jenkins-bot linux-clang-experimental-ros please
@drake-jenkins-bot linux-gcc-experimental-matlab-ros please

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Review status: 0 of 34 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[liangfok]

All CI checks passed. Now waiting on #3043 to be merged before submitting this PR for review.

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Review status: 0 of 34 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[liangfok]

+@amcastro-tri for feature review (you're the only non-platform reviewer who attended last Tuesday's meeting)
+@sherm1 for platform review (it's not Monday but you have direct knowledge of last Tuesday's meeting)

@drake-jenkins-bot linux-gcc-experimental-ros please
@drake-jenkins-bot linux-clang-experimental-ros please
@drake-jenkins-bot linux-gcc-experimental-matlab-ros please

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Review status: 0 of 20 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[liangfok]

@drake-jenkins-bot linux-gcc-experimental-ros please
@drake-jenkins-bot linux-clang-experimental-ros please
@drake-jenkins-bot linux-gcc-experimental-matlab-ros please

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Review status: 0 of 19 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[sherm1]

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Reviewed 15 of 25 files at r3, 2 of 4 files at r4, 1 of 11 files at r5, 1 of 3 files at r6.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[liangfok]

Thanks. I'm now waiting for @amcastro-tri's feature review.

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Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[amcastro-tri]

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Reviewed 15 of 25 files at r3, 2 of 4 files at r4, 1 of 11 files at r5.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[liangfok]

Thanks. Just waiting on CI now. It's pretty silly we need to re-run the entire pre-merge CI build just due to a CHANGELOG.md conflict.

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Review status: 18 of 19 files reviewed at latest revision, all discussions resolved, some commit checks pending.

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Comments from Reviewable