[ntp-composebox] Update session start time metric's description.

Update description for `NewTabPage.Composebox.FromNTPLoadToSessionStart`
to clarify that it measures the time from NTP navigation start, not from
when the NTP has finished loading.

This aligns its description with other metrics[1] that use the
`recordLoadDuration`[2] method, which utilizes `navigationStartTime`[3],
which is set when `NewTabPageUI::DidStartNavigation` is called[4].

[1]https://source.chromium.org/search?q=%22navigation%20start%22&ss=chromium%2Fchromium%2Fsrc:tools%2Fmetrics%2Fhistograms%2Fmetadata%2Fnew_tab_page%2F
[2]https://source.chromium.org/search?q=-file:out%20file:new_tab_page%20%22recordLoadDuration%22%20-file:test
[3]https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ui/webui/new_tab_page/new_tab_page_ui.cc;drc=a2d87d3bb571c5ce418e51b4a74145bdc37f7214;l=1047
[4]https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ui/webui/new_tab_page/new_tab_page_ui.cc;drc=d8339909e772f45502507714aed010c6ec2f605c;l=1003

Change-Id: Ic3ce86d42a1ad06d363fff6459ff0d9bc99b719a
Bug: 425692301
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6793804
Commit-Queue: Paul Adedeji <pauladedeji@google.com>
Reviewed-by: Roman Arora <romanarora@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1497646}