Add notes that doc lookup requires REPL (#52065)

Documentation lookup with `@doc`, `apropos`, and `Base.Docs.doc` is
exported by `Base`, but the actual implementation lives in the REPL.
Since the REPL is no longer included in the sysimg, these functions only
work if the REPL stdlib is loaded explicitly. This PR adds compat
admonitions in several places.

Author: sostock

base/docs/Docs.jl

@@ -51,6 +51,10 @@ You can retrieve docs for functions, macros and other objects as follows:
     @doc @time
     @doc md""
 
+!!! compat "Julia 1.11"
+    In Julia 1.11 and newer, retrieving documentation with the `@doc` macro requires that
+    the `REPL` stdlib is loaded.
+
 ## Functions & Methods
 Placing documentation before a method definition (e.g. `function foo() ...` or `foo() = ...`)
 will cause that specific method to be documented, as opposed to the whole function. Method
@@ -787,6 +791,9 @@ When `pattern` is a string, case is ignored. Results are printed to `io`.
 ```
 help?> "pattern"
 ```
+
+!!! compat "Julia 1.11"
+    In Julia 1.11 and newer, `apropos` requires that the `REPL` stdlib is loaded.
 """
 function apropos end
 
@@ -797,6 +804,9 @@ Return all documentation that matches both `binding` and `sig`.
 
 If `getdoc` returns a non-`nothing` result on the value of the binding, then a
 dynamic docstring is returned instead of one based on the binding itself.
+
+!!! compat "Julia 1.11"
+    In Julia 1.11 and newer, `Docs.doc` requires that the `REPL` stdlib is loaded.
 """
 function doc end
 

doc/src/manual/documentation.md

@@ -331,6 +331,10 @@ documentation between different versions of a function:
 @doc (@doc foo!) foo
 ```
 
+!!! compat "Julia 1.11"
+    In Julia 1.11 and newer, retrieving documentation with the `@doc` macro requires that
+    the `REPL` stdlib is loaded.
+
 Or for use with Julia's metaprogramming functionality:
 
 ```julia