Outdated note in the design doc

https://github.com/JuliaLang/JuliaSyntax.jl/blob/a6f2d1580f7bbad11822033e8c83e607aa31f100/docs/src/design.md?plain=1#L364

Unfortunately, this seems to no longer be true:
```julia
julia> JuliaSyntax.parsestmt(SyntaxNode, "@(a + b)")
ERROR: ParseError:
# Warning @ line 1:2
@(a + b)
#└─────┘ ── parenthesizing macro names is unnecessary
# Error @ line 1:8
@(a + b)
# └ ── invalid macro name
```

... Is there something we can do to re-allow this? I **really** want infix macros.

Read the linked section around line 364 of docs/src/design.md and reproduce the JuliaSyntax.parsestmt(SyntaxNode, "@(a + b)") example. Determine whether the task is to update the design note or restore support for parenthesized infix macros; completion should resolve that discrepancy and document or verify the resulting behavior.

Written by the indexing model from the issue text.