Docs-improve: os.getCurrentCompilerExe replace with clearer short-desc (#23270)

The doc for `getCurrentCompilerExe` was originally added at [this
commit](c4e3c4ca2d),
saying "`getAppFilename` at CT", and modified as "This is
`getAppFilename()`_ at compile time..." since
[this](0c2c2dca2a (diff-8ed10106605d9e0e3f28a927432acd8312e96791c96dbb126a52a7010cf4b44a))

Which means "at compile time, get result innerly from Nim compiler via
`getAppFilename`", not "get from nim programs".

Thus, the doc was confusing, only mentioning `compile time` and
`getAppFilename`

---------

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

lib/pure/os.nim

@@ -453,7 +453,7 @@ proc expandFilename*(filename: string): string {.rtl, extern: "nos$1",
       c_free(cast[pointer](r))
 
 proc getCurrentCompilerExe*(): string {.compileTime.} = discard
-  ## This is `getAppFilename()`_ at compile time.
+  ## Returns the path of the currently running Nim compiler or nimble executable.
   ##
   ## Can be used to retrieve the currently executing
   ## Nim compiler from a Nim or nimscript program, or the nimble binary