Update README.md and environment_variables.md for Arm

Now that there are newer architecture extensions used, update the
documentation to reflect this.

Also add missing empty lines after headers in environment_variables.md
to ensure the file is valid markdown.

Change-Id: I61d5616e1f815f80186440f27dd68ac5460c38b1
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/5868021
Reviewed-by: Frank Barchard <fbarchard@chromium.org>

README.md

@@ -5,7 +5,7 @@
 * Convert to RGB formats for rendering/effects.
 * Rotate by 90/180/270 degrees to adjust for mobile devices in portrait mode.
 * Optimized for SSSE3/AVX2 on x86/x64.
-* Optimized for Neon on Arm.
+* Optimized for Neon/SVE2/SME on Arm.
 * Optimized for MSA on Mips.
 * Optimized for RVV on RISC-V.
 

docs/environment_variables.md

@@ -7,9 +7,11 @@ For test purposes, environment variables can be set to control libyuv behavior.
 By default the cpu is detected and the most advanced form of SIMD is used.  But you can disable instruction sets selectively, or completely, falling back on C code.  Set the variable to 1 to disable the specified instruction set.
 
 ## All CPUs
+
     LIBYUV_DISABLE_ASM
 
 ## Intel CPUs
+
     LIBYUV_DISABLE_X86
     LIBYUV_DISABLE_SSE2
     LIBYUV_DISABLE_SSSE3
@@ -31,18 +33,26 @@ By default the cpu is detected and the most advanced form of SIMD is used.  But
     LIBYUV_DISABLE_AVXVNNIINT8
     LIBYUV_DISABLE_AMXINT8
 
-## ARM CPUs
+## Arm CPUs
 
     LIBYUV_DISABLE_NEON
+    LIBYUV_DISABLE_NEON_DOTPROD
+    LIBYUV_DISABLE_NEON_I8MM
+    LIBYUV_DISABLE_SVE
+    LIBYUV_DISABLE_SVE2
+    LIBYUV_DISABLE_SME
 
 ## MIPS CPUs
+
     LIBYUV_DISABLE_MSA
 
 ## LOONGARCH CPUs
+
     LIBYUV_DISABLE_LSX
     LIBYUV_DISABLE_LASX
 
 ## RISCV CPUs
+
     LIBYUV_DISABLE_RVV
 
 # Test Width/Height/Repeat