doc: improve onboarding-extras.md formatting

Fixes some formatting, improves some formatting, updates minor nits.

Refs: https://github.com/nodejs/node/issues/6655
PR-URL: https://github.com/nodejs/node/pull/6548
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/onboarding-extras.md

@@ -1,26 +1,24 @@
 ## Who to CC in issues
 
-* `lib/buffer`: @trevnorris
+| subsystem | maintainers |
-* `lib/child_process`: @cjihrig, @bnoordhuis, @piscisaereus
+| --- | --- |
-* `lib/cluster`: @cjihrig, @bnoordhuis, @piscisaereus
+| `lib/buffer` | @trevnorris |
-* `lib/{crypto,tls,https}`: @indutny, @shigeki, @nodejs/crypto
+| `lib/child_process` | @cjihrig, @bnoordhuis |
-* `lib/domains`: @misterdjules
+| `lib/cluster` | @cjihrig, @bnoordhuis |
-* `lib/{_}http{*}`: @indutny, @bnoordhuis, @nodejs/http
+| `lib/{crypto,tls,https}` | @nodejs/crypto |
-* `lib/net`: @indutny, @bnoordhuis, @piscisaereus, @chrisdickinson, @nodejs/streams
+| `lib/domains` | @misterdjules |
-* `lib/{_}stream{s|*}`: @nodejs/streams
+| `lib/{_}http{*}` | @indutny, @bnoordhuis, @mscdex, @nodejs/http |
-* `lib/repl`: @fishrock123
+| `lib/net` | @indutny, @bnoordhuis, @nodejs/streams |
-* `lib/timers`: @fishrock123, @misterdjules
+| `lib/{_}stream{s|*}` | @nodejs/streams |
-* `lib/zlib`: @indutny, @bnoordhuis
+| `lib/repl` | @fishrock123 |
-
+| `lib/timers` | @fishrock123, @misterdjules |
-* `src/async-wrap.*`: @trevnorris
+| `lib/zlib` | @indutny, @bnoordhuis |
-* `src/node_crypto.*`: @indutny, @shigeki, @nodejs/crypto
+| `src/async-wrap.*` | @trevnorris |
-
+| `src/node_crypto.*` | @nodejs/crypto |
-* `test/*`: @nodejs/testing, @trott
+| `test/*` | @nodejs/testing |
-
+| `tools/eslint`, `.eslintrc` | @silverwind, @trott |
-* `tools/eslint`, `.eslintrc`: @silverwind, @trott
+| upgrading v8 | @bnoordhuis, @targos, @ofrobots |
-
+| upgrading npm | @thealphanerd, @fishrock123 |
-* upgrading v8: @bnoordhuis / @targos / @ofrobots
-* upgrading npm: @thealphanerd, @fishrock123
 
 
 When things need extra attention, are controversial, or `semver-major`: @nodejs/ctc
@@ -52,6 +50,8 @@ Please use these when possible / appropriate
 * `feature request` - Any issue that requests a new feature (usually not PRs)
 * `good first contribution` - Issues suitable for newcomers to process
 
+--
+
 * `semver-{minor,major}`
   * be conservative – that is, if a change has the remote *chance* of breaking something, go for semver-major
   * when adding a semver label, add a comment explaining why you're adding it
@@ -94,7 +94,7 @@ to update from nodejs/node:
   * prefer to make the originating user update the code, since they have it fresh in mind
 * first, reattempt with `git am -3` (3-way merge)`
 * if `-3` still fails, and you need to get it merged:
-  * `git fetch origin pull/N/head:pr-N && git checkout pr-N && git rebase master`
+  * `git fetch upstream pull/N/head:pr-N && git checkout pr-N && git rebase master`
 
 
 ## best practices