[Doc] Update docs to clarify cache-related environment variables. (in…

…tel#15795)

We have two caches: in-memory and persistent. For some cache-related
environment variables, it is not explicitly clear whether they are
applicable for in-memory cache or persistent. So, this PR clarifies the
description of cache-related environment variables.

Side note: I see that we never use `SYCL_CACHE_EVICTION_DISABLE`,
`SYCL_CACHE_MAX_SIZE`, and `SYCL_CACHE_THRESHOLD` in the code base, yet
we have them in the documentation. I think, for now, we can just leave
them as it is since I'm working on implementing cache eviction and I can
reuse them in future.

sycl/doc/EnvironmentVariables.md

@@ -14,11 +14,11 @@ compiler and runtime.
 | `SYCL_CACHE_DISABLE_PERSISTENT (deprecated)` | Any(\*) | Has no effect. |
 | `SYCL_CACHE_PERSISTENT` | Integer | Controls persistent device compiled code cache. Turns it on if set to '1' and turns it off if set to '0'. When cache is enabled SYCL runtime will try to cache and reuse JIT-compiled binaries. Default is off. |
 | `SYCL_CACHE_IN_MEM` | '1' or '0' | Enable ('1') or disable ('0') in-memory caching of device compiled code. When cache is enabled SYCL runtime will try to cache and reuse JIT-compiled binaries. Default is '1'. |
-| `SYCL_CACHE_EVICTION_DISABLE` | Any(\*) | Switches cache eviction off when the variable is set. |
-| `SYCL_CACHE_MAX_SIZE` | Positive integer | Cache eviction is triggered once total size of cached images exceeds the value in megabytes (default - 8 192 for 8 GB). Set to 0 to disable size-based cache eviction. |
-| `SYCL_CACHE_THRESHOLD` | Positive integer | Cache eviction threshold in days (default value is 7 for 1 week). Set to 0 for disabling time-based cache eviction. |
-| `SYCL_CACHE_MIN_DEVICE_IMAGE_SIZE` | Positive integer | Minimum size of device code image in bytes which is reasonable to cache on disk because disk access operation may take more time than do JIT compilation for it. Default value is 0 to cache all images. |
-| `SYCL_CACHE_MAX_DEVICE_IMAGE_SIZE` | Positive integer | Maximum size of device image in bytes which is cached. Too big kernels may overload disk too fast. Default value is 1 GB. |
+| `SYCL_CACHE_EVICTION_DISABLE` | Any(\*) | Switches persistent cache eviction off when the variable is set. |
+| `SYCL_CACHE_MAX_SIZE` | Positive integer | Persistent cache eviction is triggered once total size of cached images exceeds the value in megabytes (default - 8 192 for 8 GB). Set to 0 to disable size-based cache eviction. |
+| `SYCL_CACHE_THRESHOLD` | Positive integer | Persistent cache eviction threshold in days (default value is 7 for 1 week). Set to 0 for disabling time-based cache eviction. |
+| `SYCL_CACHE_MIN_DEVICE_IMAGE_SIZE` | Positive integer | Minimum size of device code image in bytes which is reasonable to cache on disk because disk access operation may take more time than do JIT compilation for it. Applicable only for persistent cache. Default value is 0 to cache all images. |
+| `SYCL_CACHE_MAX_DEVICE_IMAGE_SIZE` | Positive integer | Maximum size of device image in bytes which is cached. Caching big kernels may overload the disk very fast. Applicable only for persistent cache. Default value is 1 GB. |
 | `SYCL_ENABLE_DEFAULT_CONTEXTS` | '1' or '0' | Enable ('1') or disable ('0') creation of default platform contexts in SYCL runtime. The default context for each platform contains all devices in the platform. Refer to [Platform Default Contexts](extensions/supported/sycl_ext_oneapi_default_context.asciidoc) extension to learn more. Enabled by default on Linux and disabled on Windows. |
 | `SYCL_RT_WARNING_LEVEL` | Positive integer | The higher warning level is used the more warnings and performance hints the runtime library may print. Default value is '0', which means no warning/hint messages from the runtime library are allowed. The value '1' enables performance warnings from device runtime/codegen. The values greater than 1 are reserved for future use. |
 | `SYCL_USM_HOSTPTR_IMPORT` | Integer | Enable by specifying non-zero value. Buffers created with a host pointer will result in host data promotion to USM, improving data transfer performance. To use this feature, also set SYCL_HOST_UNIFIED_MEMORY=1. |