Fix docs shm calculator (#24649)

* match the docs shm calculator to the backend's minimum

The calculator left out the room Frigate reserves for 2 cameras added at runtime, counted the per-frame overhead once per camera instead of per frame, and reserved 40mb instead of 50mb for logs and the nginx cache, so a setup sized with it got well under 20 frames and the "SHM size is too small" warning. It now computes the same minimum as calculate_shm_requirements() and also shows the size for the full 50 frame cache.

* size the docs shm calculator for 25 frames

Sizing to the backend's 20 frame minimum could land at 19 frames from float rounding and still show the warning.

docs/docs/frigate/installation.md

@@ -78,9 +78,9 @@ Users of the Snapcraft build of Docker cannot use storage locations outside your
 
 Frigate utilizes shared memory to store frames during processing. The default `shm-size` provided by Docker is **64MB**.
 
-The default shm size of **128MB** is fine for setups with **2 cameras** detecting at **720p**. If Frigate is exiting with "Bus error" messages, it is likely because you have too many high resolution cameras and you need to specify a higher shm size, using [`--shm-size`](https://docs.docker.com/engine/reference/run/#runtime-constraints-on-resources) (or [`service.shm_size`](https://docs.docker.com/compose/compose-file/compose-file-v2/#shm_size) in Docker Compose). If raising the shm size does not help, check your [process and file limits](#process-and-file-limits) as well.
+Frigate needs about **210MB** for **2 cameras** detecting at **720p**. If Frigate is exiting with "Bus error" messages, it is likely because you have too many high resolution cameras and you need to specify a higher shm size, using [`--shm-size`](https://docs.docker.com/engine/reference/run/#runtime-constraints-on-resources) (or [`service.shm_size`](https://docs.docker.com/compose/compose-file/compose-file-v2/#shm_size) in Docker Compose). If raising the shm size does not help, check your [process and file limits](#process-and-file-limits) as well.
 
-The Frigate container also stores logs in shm, which can take up to **40MB**, so make sure to take this into account in your math as well.
+The Frigate container also stores logs and an nginx cache in shm, which can take up to **50MB**. The calculator below includes this, along with room for 2 cameras added while Frigate is running. Its result holds about 25 frames per camera, a little above the 20 frame minimum Frigate warns below. Frigate keeps up to 50 when there is room, which gives slow processes more time to read each frame.
 
 <ShmCalculator/>