Additional SVG diagrams in updaters package.md

[abyrd]

This PR adds more diagrams and explanation to the src/main/java/org/opentripplanner/updater/package.md developer documentation. The diagrams are stored as Excalidraw files, which have been rendered to SVG in the images subdirectory. The new diagrams are:

1. A high-level diagram of how the TimetableSnapshot is currently used to look up Timetables/TripTimes for a TripPattern.
2. A step-by-step sequence showing how successive timetable snapshots are built up in a buffer.

The second, larger diagram also shows how snapshots interact with multiple routing and API requests, and the effects of garbage collection. This information is complementary to the other SVG sequence diagram already present in this directory.

I also included a few edits to Javadoc on a topic that I ran into while working on this (Transmodel "via" strings).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.47%. Comparing base (fd222ec) to head (bbe4e55). Report is 23 commits behind head on dev-2.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev-2.x #5936 +/- ## ============================================= + Coverage 69.45% 69.47% +0.01% - Complexity 17066 17077 +11 ============================================= Files 1934 1937 +3 Lines 73624 73679 +55 Branches 7539 7539 ============================================= + Hits 51134 51185 +51 - Misses 19865 19870 +5 + Partials 2625 2624 -1 ```

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