docs: bpf: map_array: Specify BPF_MAP_TYPE_PERCPU_ARRAY value size limit

Specify value size limit for BPF_MAP_TYPE_PERCPU_ARRAY which
is PCPU_MIN_UNIT_SIZE (32 kb). In percpu allocator (mm: percpu),
any request with a size greater than PCPU_MIN_UNIT_SIZE is rejected.

Signed-off-by: Alex Tran <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

Documentation/bpf/map_array.rst

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 creation time. All array elements are pre-allocated and zero initialized when
 created. ``BPF_MAP_TYPE_PERCPU_ARRAY`` uses a different memory region for each
 CPU whereas ``BPF_MAP_TYPE_ARRAY`` uses the same memory region. The value
-stored can be of any size, however, all array elements are aligned to 8
-bytes.
+stored can be of any size for ``BPF_MAP_TYPE_ARRAY`` and not more than
+``PCPU_MIN_UNIT_SIZE`` (32 kB) for ``BPF_MAP_TYPE_PERCPU_ARRAY``. All
+array elements are aligned to 8 bytes.
 
 Since kernel 5.5, memory mapping may be enabled for ``BPF_MAP_TYPE_ARRAY`` by
 setting the flag ``BPF_F_MMAPABLE``. The map definition is page-aligned and