Add motivation for `error` element in `method`

Add paragraphs to the `error` sub section under `method` that explains why `error` has been specified next to the similar `out` argument.

- **Static code analysis**
A service spec can be checked for error value coverage, flagging methods that lack minimal error handling.

- **Consistent error management**
The generated code handles all errors in the same distinct way, removing the risk of having a service developer mixing return values and error reporting in an arbitrary way.

- **Exception support**
An explicit error element maps directly to raised exceptions in languages that support them.

- **Optional**
An error section does not need to be specified for a method. Conversely, language-specific code generator may chose to emit error reporting code as another return value in cases where the language offers limited error management (such as C).

Start in the IFEX documentation at the `method` section and locate its `error` subsection. Add the requested motivation paragraphs covering static analysis, consistent error management, exception support, and optional language-specific handling; done means all four points are clearly documented alongside the existing `out` explanation.

Written by the indexing model from the issue text.