doc: warn users that PATCHES may not work properly when CPM_SOURCE_CACHE is unset.

[lemire]

Issue https://github.com/cpm-cmake/CPM.cmake/issues/577 is difficult to debug once triggered given that it is undocumented. It is likely to affect virtually all developers who rely on PATCHES with CPM_SOURCE_CACHE unset. If there is no plan to resolve it quickly, and if the assumption is that the users should set CPM_SOURCE_CACHE when they use PATCHES, then it should be documented as such.