doc: improve onboarding-extras.md formatting

Fixes some formatting, improves some formatting, updates minor nits.
Refs: #6655
PR-URL: #6548
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: Fishrock123

doc/onboarding-extras.md

@@ -1,26 +1,24 @@
 ## Who to CC in issues
 
-*`lib/buffer`: @trevnorris
-*`lib/child_process`: @cjihrig, @bnoordhuis, @piscisaereus
-*`lib/cluster`: @cjihrig, @bnoordhuis, @piscisaereus
-*`lib/{crypto,tls,https}`: @indutny, @shigeki, @nodejs/crypto
-*`lib/domains`: @misterdjules
-*`lib/{_}http{*}`: @indutny, @bnoordhuis, @nodejs/http
-*`lib/net`: @indutny, @bnoordhuis, @piscisaereus, @chrisdickinson, @nodejs/streams
-*`lib/{_}stream{s|*}`: @nodejs/streams
-*`lib/repl`: @fishrock123
-*`lib/timers`: @fishrock123, @misterdjules
-*`lib/zlib`: @indutny, @bnoordhuis
-
-*`src/async-wrap.*`: @trevnorris
-*`src/node_crypto.*`: @indutny, @shigeki, @nodejs/crypto
-
-*`test/*`: @nodejs/testing, @trott
-
-*`tools/eslint`, `.eslintrc`: @silverwind, @trott
-
-* upgrading v8: @bnoordhuis / @targos / @ofrobots
-* upgrading npm: @thealphanerd, @fishrock123
+| subsystem | maintainers |
+| --- | --- |
+|`lib/buffer`|@trevnorris|
+|`lib/child_process`|@cjihrig, @bnoordhuis|
+|`lib/cluster`|@cjihrig, @bnoordhuis|
+|`lib/{crypto,tls,https}`|@nodejs/crypto|
+|`lib/domains`|@misterdjules|
+|`lib/{_}http{*}`|@indutny, @bnoordhuis, @mscdex, @nodejs/http|
+|`lib/net`|@indutny, @bnoordhuis, @nodejs/streams|
+| `lib/{_}stream{s|*}` |@nodejs/streams|
+|`lib/repl`|@fishrock123|
+|`lib/timers`|@fishrock123, @misterdjules|
+|`lib/zlib`|@indutny, @bnoordhuis|
+|`src/async-wrap.*`|@trevnorris|
+|`src/node_crypto.*`|@nodejs/crypto|
+|`test/*`|@nodejs/testing|
+|`tools/eslint`, `.eslintrc`|@silverwind, @trott|
+| 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