Warn that ExternalFallbacks requires Documenter 1.3

docs/src/fallback.md

@@ -1,7 +1,7 @@
 # Fallback Resolution
 
 !!! warning
-    The [`ExternalFallbacks`](@ref) plugin described here is experimental.
+    The [`ExternalFallbacks`](@ref) plugin described here is available only with `Documenter >= v1.3.0` and should be considered experimental.
 
 In some situations, you may need to include a docstring from an external package in your documentation – for example, you you are extending a function from that package, you may want to show the function docstring. If that docstrings contains an `@ref` link, you have a problem: that link is resolvable in the external documentation, but not in *your* documentation.
 

src/fallback.jl

@@ -4,7 +4,8 @@ import Documenter: XRefResolvers, Selectors, xref_unresolved
 """Plugin for letting `@ref` links fall back to `@extref` links.
 
 !!! warning
-    This plugin is experimental.
+    This plugin is available only with `Documenter >= v1.3.0` and should be
+    considered as experimental.
 
 ```julia
 fallbacks = ExternalFallbacks(pairs...)

test/test_expand_extrefs.jl

@@ -12,14 +12,14 @@ function parse_md_link(text)
     mdpage = Markdown.parse(text)
     paragraph = first(convert(MarkdownAST.Node, mdpage).children)
     if length(paragraph.children) != 1
-        @error "citation $(repr(text)) must parse into a single MarkdownAST.Link" ast =
+        @error "link text $(repr(text)) must parse into a single MarkdownAST.Link" ast =
             collect(paragraph.children)
-        error("Invalid citation: $(repr(text))")
+        error("Invalid link text: $(repr(text))")
     end
     link = first(paragraph.children)
     if !(link.element isa MarkdownAST.Link)
         @error "citation $(repr(text)) must parse into MarkdownAST.Link" ast = link
-        error("Invalid citation: $(repr(text))")
+        error("Invalid link text: $(repr(text))")
     end
     return link
 end