Add limitations section to Linking.md to clarify element and data seg…

…ment indices renumbering is not supported (WebAssembly#193)

Linking.md

@@ -24,7 +24,7 @@ tasks need to be performed:
 - Merging of globals sections (re-numbering globals)
 - Merging of event sections (re-numbering events)
 - Merging of table sections (re-numbering tables)
-- Merging of data segments (re-positioning data)
+- Merging of data segments (re-positioning data with [limitations](#limitations))
 - Resolving undefined external references
 - Synthesizing functions to call constructors and perform other initialization
 
@@ -686,3 +686,12 @@ have its address computed as follows:
 
 The variable can then be used as normal. Upon thread exit, the runtime should free
 the memory allocated for the TLS block.
+
+### Limitations
+
+Note data segments are only re-positioned, i.e. only the offsets of active data
+elements can be updated. There is no relocation type for data segment indices
+such that instructions like `data.drop` or `memory.init` are not supported.
+
+Similarly, there is no relocation type for element segment indices and the
+instructions `elem.drop` and `table.init` are not supported.