[Doc] Refactor descriptions of `min-legal-vector-width`

[pengfei]

Summary

Based on comments from @arsenm.

[arsenm]

llvm/docs/LangRef.rst

2236–2237

The first sentence isn't grammatically correct. Also should not start out with the word legal

2239

Still not stating what legal means in this context

2240

Grammar?

2241

return types?

Also what is the behavior for aggregates? Aggregates containing vectors? Nested aggregates with vectors?

2243

Returnings isn't a word; "return values"?

2244–2245

We can't spec attributes in terms of what clang's behavior is today

[pengfei]

I want to remove the description from LangRef directly since we decided to only support this attribute in X86. WDYT?

llvm/docs/LangRef.rst

2241

Good question. I think we have considered such cases. See https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CGCall.cpp#L4729-L4740

[pengfei]

Move doc to comments.

[pengfei]

clang/lib/CodeGen/CodeGenFunction.cpp

502–504 ↗

(On Diff #484805)

I still think the sentence is ill-formed. But I have tried my best. Welcome for suggestions.

llvm/docs/LangRef.rst

``"warn-stack-size"="<threshold>"``
     duplicate definitions are linked together with differing values.
 ``vscale_range(<min>[, <max>])``
     This attribute indicates the minimum and maximum vscale value for the given
     function. The min must be greater than 0. A maximum value of 0 means
     unbounded. If the optional max value is omitted then max is set to the
     value of min. If the attribute is not present, no assumptions are made
     about the range of vscale.
 ``"min-legal-vector-width"="<size>"``
-    This attribute indicates the minimum legal vector width required by the
-    calling convension. It is the maximum width of vector arguments and
-    returnings in the function and functions called by this function. Because
-    all the vectors are supposed to be legal type for compatibility.
-    Backends are free to ignore the attribute if they don't need to support
-    different maximum legal vector types or such information can be inferred by
-    other attributes.
+    This attribute indicates the minimum vector width in bits that code
+    generator must make them legal. It is required by the target’s ABI.
+    A target like X86 may dynamically decide vector types in the function are
+    legal or not according to optimizations. When this attribute is set, code
+    generator will not turn vector types with the same or below size illegal.
+    The value is calculated from the vector arguments and returnings in the
+    function and functions directly called by this function.
+    A value 0 means no vector types in arguments and returnings in the function
+    as well as its callees. Lack of the attribute means the IR didn't come from
+    clang and can't be trusted to have been audited.
+    This attribute doesn't guarantee arbitrary vector width specified will be
+    treated as legal type in code generator. Users are not encouraged to
+    directly pass or return vector types out of the capacity of their targets.
 
 Call Site Attributes
 ----------------------
 
 In addition to function attributes the following call site only
 attributes are supported:
 
 ``vector-function-abi-variant``