Document about noinline calling convention and exportcpp pragma in Ni…

…m manual (#24323)

It seems exportcpp was implemented in v1.0 but there is no documentation
about it excepts changelog.
`noinline` is used in many procedures in Nim code but there is also no
documentation about it.

---------

Co-authored-by: Andreas Rumpf <[email protected]>

doc/manual.md

@@ -2241,6 +2241,10 @@ Nim supports these `calling conventions`:idx:\:
     only a hint for the compiler: it may completely ignore it, and
     it may inline procedures that are not marked as `inline`.
 
+`noinline`:idx:
+:   The backend compiler may inline procedures that are not marked as `inline`.
+    The noinline convention prevents it.
+
 `fastcall`:idx:
 :   Fastcall means different things to different C compilers. One gets whatever
     the C `__fastcall` means.
@@ -8646,6 +8650,14 @@ pragma should be used in addition to the `exportc` pragma. See
 [Dynlib pragma for export].
 
 
+Exportcpp pragma
+----------------
+The `exportcpp` pragma works like the `exportc` pragma but it requires the `cpp` backend.
+When compiled with the `cpp` backend, the `exportc` pragma adds `export "C"` to
+the declaration in the generated code so that it can be called from both C and
+C++ code. `exportcpp` pragma doesn't add `export "C"`.
+
+
 Extern pragma
 -------------
 Like `exportc` or `importc`, the `extern` pragma affects name