Added rustdoc documentation to compiler docs.

[davidtwco]

Fixes #50004.

r? @alexcrichton
(since you reviewed the last PR about compiler docs)

[alexcrichton]

@bors: r+

[bors]

📌 Commit 9b02379 has been approved by alexcrichton

[bors]

⌛ Testing commit 9b02379becdd8c4f9359ee7b97304198d5878b10 with merge ba3866e26492642a9a809180fd15888b62a77e80...

[bors]

💔 Test failed - status-travis

[rust-highfive]

The job dist-x86_64-linux-alt of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[01:00:57] 
[01:00:58]     Finished release [optimized] target(s) in 2m 7.10s
[01:00:58] [TIMING] Rustc { stage: 2, target: "x86_64-unknown-linux-gnu" } -- 127.154
[01:00:58] Documenting stage2 rustdoc (x86_64-unknown-linux-gnu)
[01:00:58] thread 'main' panicked at 'symlink_dir_force(&builder.config, &out, &out_dir) failed with No such file or directory (os error 2)', bootstrap/doc.rs:798:9
[01:00:58] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap dist --host x86_64-unknown-linux-gnu --target x86_64-unknown-linux-gnu
[01:00:58] Build completed unsuccessfully in 0:55:49
travis_time:end:09874a25:start=1526822543684853229,finish=1526826202643690318,duration=3658958837089

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[rust-highfive]

The job dist-x86_64-linux-alt of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[01:00:57] 
[01:00:58]     Finished release [optimized] target(s) in 2m 7.10s
[01:00:58] [TIMING] Rustc { stage: 2, target: "x86_64-unknown-linux-gnu" } -- 127.154
[01:00:58] Documenting stage2 rustdoc (x86_64-unknown-linux-gnu)
[01:00:58] thread 'main' panicked at 'symlink_dir_force(&builder.config, &out, &out_dir) failed with No such file or directory (os error 2)', bootstrap/doc.rs:798:9
[01:00:58] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap dist --host x86_64-unknown-linux-gnu --target x86_64-unknown-linux-gnu
[01:00:58] Build completed unsuccessfully in 0:55:49
travis_time:end:09874a25:start=1526822543684853229,finish=1526826202643690318,duration=3658958837089

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[davidtwco]

Oops! Forgot to make sure that directory exists, testing a fix locally. Also, that bot is great.

[davidtwco]

@alexcrichton would you be able to take another look at this now I've fixed the previous bug?

[alexcrichton]

@bors: r+

[bors]

📌 Commit 52b83d7 has been approved by alexcrichton

[bors]

⌛ Testing commit 52b83d7 with merge d0456c6...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing d0456c6 to master...