whence: add page (#4021)

pages/osx/whence.md

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+# whence
+
+> A zsh builtin to indicate how a given command would be interpreted.
+
+- Interpret {{command}}, with expansion if defined as an `alias` (similar to the `command -v` builtin):
+
+`whence {{command}}`
+
+- Display type of {{command}}, with location if defined as a function, or binary (equivalent to the `type` and `command -V` builtins):
+
+`whence -v {{command}}`
+
+- Same as above, except display content of shell functions instead of location (equivalent to `which` builtin):
+
+`whence -c {{command}}`
+
+- Same as above, but show all occurrences on command path (equivalent to the `where` builtin):
+
+`whence -ca {{command}}`
+
+- Search only the `PATH` for {{command}}, ignoring builtins, aliases or shell functions (equivalent to the `where` command):
+
+`whence -p {{command}}`