Restore SkFontMetrics documentation.
Restore SkFontMetrics documentation.

With 462505f3100f0abc52adfd4506b62719e7151e18 "include refresh" most of
the useful parts of the SkFontMetrics documentation were removed.
Restore the documentation and update it with the latest information.

Change-Id: I28c13bc194b53265818e31e7b26e93a390f6259c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/234945
Reviewed-by: Hal Canary <fd9641b23487c8ee78406c27513624c460600766@google.com>
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

HalCanary/skia-hc,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia