AW docs: clarify commandline flag support

No change to logic, only docs.

This clarifies WebView's flag support. Namely, it explains that we don't
"support" all flags, but that we support toggling them (and the behavior
itself may or may not be stable). This also links to the layer diagram.

This clarifies some of the language around "features," defining
"base::Feature" and updating some references to use either
"base::Feature" or "Feature" rather than "feature" (which could be
mistaken for a general enhancement to the product).

Bug: 918221
Test: tools/md_browser/md_browser.py
Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I570eeed005b8671a003028e0c45b8ac03d37fc71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2018151
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Shimi Zhang <ctzsm@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734717}

android_webview/docs/commandline-flags.md

@@ -78,15 +78,29 @@ WebView app.
 
 ## Applying Features with flags
 
-WebView supports the same `--enable-features=feature1,feature2` and
-`--disable-features=feature3,feature4` syntax as the rest of Chromium. You can
-use these like any other flag. Please consult
-[`base/feature_list.h`](https://cs.chromium.org/chromium/src/base/feature_list.h)
-for details.
+[`base::Feature`s](/base/feature_list.h) (or, "Features") are Chromium's
+mechanism for toggling off-by-default code paths. While debugging flags are also
+off-by-default, Features typically guard soon-to-launch product enhancements
+until they're tested enough for field trials or public launch, at which point
+the Feature is removed and the legacy code path is no longer supported and
+removed from the codebase. On the other hand, debugging flags don't "launch," as
+they're typically only helpful for debugging issues.
+
+WebView supports the same syntax for toggling Features as the rest of chromium:
+`--enable-features=feature1,feature2` and
+`--disable-features=feature3,feature4`. You can apply `--enable-features` and
+`--disable-features` like any other flags, per the steps above. Please consult
+[`base/feature_list.h`](/base/feature_list.h) for details.
+
+## Finding Features and flags
+
+WebView supports toggling any flags/Features supported in any layer we
+depend on (ex. content). For more details on Chromium's layer architecture, see
+[this diagram](https://www.chromium.org/developers/content-module) (replace
+"chrome" with "android\_webview"). Although we support toggling these flags, not
+all flags will have an effect when toggled, nor do we guarantee WebView
+functions correctly when the flag is toggled.
 
-## Interesting flags
-
-WebView supports any flags supported in any layer we depend on (ex. content).
 Some interesting flags and Features:
 
  * `--show-composited-layer-borders`: highlight rendering layers, which is