Improve docs around AllocationLimitMegabytes

[antonfirsov]

Noting down a couple of improvement ideas after re-reading some docs I made.

• Link the AllocationLimitMegabytes property from the security conceptual docs:

- Imposing conservative allocation limits by configuring a custom MemoryAllocator
+ Imposing conservative allocation limits by configuring a custom MemoryAllocator with [`MemoryAllocationOptions.AllocationLimitMegabytes`](https://docs.sixlabors.com/api/ImageSharp/SixLabors.ImageSharp.Memory.MemoryAllocatorOptions.html#SixLabors_ImageSharp_Memory_MemoryAllocatorOptions_AllocationLimitMegabytes) set to a value lower value.

• Link back the security docs from the property docs.
• For both the MaximumPoolSizeMegabytes docs and AllocationLimitMegabytes can be more precise about defining the default value:

- null means platform default: 1GB on 32-bit processes, 4GB on 64-bit processes.
+ The value is null by default, which means platform defaults will be used: 1GB on 32-bit processes, 4GB on 64-bit processes.