Replace "entry" with "element" in function docs

[owen-mc-diffblue]

"Entry" has a specific meaning in this file, because of entryt. The
function docs for object_map_dt use it to mean an element in a map.
Change them to use "element" instead.

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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[chrisr-diffblue]

I don't know this code, so someone more familiar than me should probably also take a look, but these docs changes seem reasonable to me.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: e5991c7).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/102538204