Updated Physically Based Sky documentation with more warnings about warmup cost.

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -85,6 +85,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Support for encoded HDR cubemaps, configurable via the HDR Cubemap Encoding project setting.
 - The rendering order of decals that have a similar draw order value was modified. The new order should be the reverse of the previous order.
 - Render Graph object pools are now cleared with render graph cleanup
+- Updated Physically Based Sky documentation with more warnings about warmup cost.
 
 ## [13.1.2] - 2021-11-05
 

com.unity.render-pipelines.high-definition/Documentation~/Override-Physically-Based-Sky.md

@@ -132,7 +132,21 @@ The default values in either mode make it so the planet's surface is at **0** on
 
 ## Warmup performance impact
 
-HDRP has to perform heavy precomputation to render this sky type. Because of this, the first few frames (depending on the *Number of bounces* parameter) take much longer to render. Take this into consideration when switching from another sky type to the Physically Based Sky for example as it might produce a noticeable drop in frame rate.
+When you switch to or from a Physically Based Sky, it might cause a noticeable drop in framerate. This is because HDRP performs a large amount of precomputations to render a Physically Based Sky, so the first few frames (depending on the Number of bounces parameter) takes more time to render than other HDRP sky types.
+This also applies when HDRP uses the volume system to interpolate between two different Physically Based Skies with different sets of parameters. To do this, HDRP restarts the precomputation every frame in which it performs interpolation. This causes a noticeable drop in framerate. To avoid this, use a single set of Physically Based Sky parameters for a scene and change the sun light direction and intensity to achieve the result you want.
+
+HDRP restarts precomputation when you change the following parameters:
+- Type
+- Planetary Radius
+- Ground Tint
+- Air Maximum Altitude
+- Air Density
+- Air Tint
+- Aerosol Maximum Altitude
+- Aerosol Density
+- Aerosol Tint
+- Aerosol Anisotropy
+- Number of Bounces
 
 ### Reference list