Explain entry/prefix values may be restricted

Some existing codegens (notably ELFv2+GCC+PPC) can only do some values
of `prefix`/`entry`. Call out that it is possible for some values to
cause errors, even if the target/backend combination supports
patchability.

Author: maurer

text/3543-patchable-function-entry.md

@@ -79,15 +79,15 @@ This flag sets the default nop padding for all functions in the crate. In most c
 
 `prefix` is calculated as `offset`. `entry` is calculated as `nop_count - offset`. This unusual mode of specification is intended to mimic the compiler flags of `clang` and `gcc` for ease of build system integration.
 
-Specifying the compiler flag for a backend or architecture which does not support this feature will result in an error.
+Specifying the compiler flag for a backend or architecture which does not support this feature will result in an error. Some backend / architecture combinations may only support some values of `entry` and `prefix`, in which case an error will also be generated for invalid values.
 
 ## Attributes `#[patchable]` and `#[unpatchable]`.
 
 This attribute allows specification of either the `prefix` or `entry` values or both, using the format `#[patchable(prefix = m, entry = n)]`. If either is left unspecified, it overrides them to a default value of 0. Specifying neither prefix nor entry is an error, but explicitly setting them both to 0 is allowed.
 
 As this is specified via an attribute, it will persist across crate boundaries unlike the compiler flag.
 
-Specifying any amount of padding other than 0 in an attribute will result in an error on backends or architectures which do not support this feature.
+Specifying any amount of padding other than 0 in an attribute will result in an error on backends or architectures which do not support this feature. Some architecture/backend combinations may only support a subset of prefix and entry nop counts, and may generate errors when other counts are requested.
 
 `#[unpatchable]` is a built-in alias for `#[patchable(prefix = 0, entry = 0)]`.